This improves job captions, the grouping of logs underneath them, and
the handling of import errors. It also adds a number of log-related
utilities to help achieve this.
**Key Changes:**
* Job captions for facets now include the name of the object (e.g.,
module, library, facet). A caption has also been added to the top-level
build of imports (e.g., for the server and `lake lean`).
* Stray I/O and errors outside the build job in a build function
captioned with `withRegisterJob` (e.g., user-defined targets) will now
be properly grouped under that caption instead of ending up under
"Computing build jobs". Stray I/O will be converted to a single
informational log entry.
* Builds no longer fail immediately on erroneous imports. Lake will now
attempt to recover as best as possible from any import errors.
Information on the import error will appear both in the build of the
erroneous import and in the files which transitive import it. For
example, uf `Lib.B` imports a missing module `Lib.A`, then the build of
`Lib.A` will mention that the file does not exist, and the build of
`Lib.B` will mention the bad import of `Lib.A`.
Closes#3351. Closes#3809.
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