Error detail/hint style fixup

When fixing review comments for error message style on another patch, I noticed
that there were a few error details/hints/contexts that weren’t following the
style guide in the documentation. This might be intentional from when they
were added, or we intentionally avoid changing after the fact to not impact
translations too much, or we just don’t care (didn’t find any past mention in
the archives though), but my OCD cannot unsee it now so I figured I'd send it
and see.

Attached patch ensures that (i) details and hints have leading capitalization,
have double spaces after punctuation and ends with period; (ii) context should
not be capitalized and should not end with period; (iii) test .out files match
the changes.

cheers ./daniel