Define a generic "Failed" case for app-level errors (and a few others).

We were previously exporting root causes from one layer of abstraction to the next. For example, anything that called into the database could cause an `sqlx::Error`, so anything that transitively called into that logic exported a `Database(sqlx::Error)` error variant of its own, using `From` to map errors from inner type to outer type.

This had a couple of side effects. First, it required each layer of error handling to carry with it a `From` implementation unwrapping and rewrapping root causes from the next layer down. This was particularly apparent in the event and boot endpoints, which had separate error cases unique to crypto key processing errors solely because they happened to involve handling events that contained those keys. There were others, including the pervasive `Database(sqlx::Error)` error variants.

Separately, none of the error variants introduced for this purpose were being used for anything other than printing to stderr. All the complexity of From impls and all the structure of the error types was being thrown away at top-level error handlers.

This change replaces most of those error types with a generic `Failed` error. A `Failed` carries with it two pieces of information: a (boxed) underlying error, of any boxable `Error` type, and text meant to explain the context and cause of an error. Code which acts on errors can treat `Failed` as a catch-all case, while individually handling errors that signify important cases. Errors can be moved into or out of the `Failed` case by refactoring, as needed.

The design of `Failed` is heavily motivated by [anyhow's `context` system][context] as a way for the programmer to capture immediate intention as an explanation for some underlying error. However, instead of accepting the full breadth of types that implement `Display`, a `Failed` can only carry strings as explanation. We don't need the generality at this time, and the implementation underlying it is pretty complex for what it does.

[context]: https://docs.rs/anyhow/latest/anyhow/struct.Error.html#method.context

This change also means that the full source chain for an error is now available to top-level error handlers, allowing more complete error messages. For example, starting `pilcrow` with an invalid network listen address produces

    Failed to bind to www.google.com:64209

    Caused by:
      Can't assign requested address (os error 49)

instead of the previous

    Error: Io(Os { code: 49, kind: AddrNotAvailable, message: "Can't assign requested address" })

which previously captured the same _cause_, but without the formatting (see previous commit) and without the _context_ (this commit). Similar improvements are available for many of the error scenarios Pilcrow is designed to give up on.

When deciding which errors to use `Failed` with, I've used the heuristic that if something can fail for more than one underlying reason, and if the caller will only ever need to be able to differentiate those reasons after substantial refactoring anyways, then the reasons should collase into `Failed`. If there's either only a single underlying failure reason possible, or only errors arising out of the function body possible, then I've left error handling alone.

In the process I've refactored most request-handler-level error mappings to explicitly map `Failed` to `Internal`, rather than having a catch-all mapping for all unhandled errors, to make it easier to remember to add request-level error representations when adding app-level error cases.

This also includes helper traits for `Error` and `Result`, to make constructing `Failed` (and errors that include `Failed` as an alternative) easier to do, and some constants for the recurring error messages related to transaction demarcation. I'm not completely happy with the repetitive nature of those error cases, but this is the best I've arrived at so far.

As errors are no longer expected to be convertible up the call stack, the `NotFound` and `Duplicate` helper traits for database errors had to change a bit. Those previously assumed that they would be used in the context of an error type implementing `From<sqlx::Error>` (or from another error type with similar characteristics), and that's not the case any more. The resulting idiom for converting a missing value into a domain error is `foo.await.optional().fail(MESSAGE)?.ok_or(DOMAIN ERROR)?`, which is rather clunky, but I've opted not to go further with it. The `Duplicate` helper is just plain gone, as it's not easily generalizable in this structure and using `match` is more tractable for me.

Finally, I've changed the convention for error messages from `all lowercase messages in whatever tense i feel like at the moment` to `Sentence-case messages in the past tense`, frequently starting with `Failed to` and a short summary of the task at hand. This, as above, makes error message capitalization between Pilcrow's own messages and messages coming from other libraries/the Rust stdlib much more coherent and less jarring to read.

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