This changes to an `Error` return to let the driver do the "error: "
prefix, except for one case with `help` that needs more work to change
(I'm not planning on picking up that TODO). It also changes
capitalization, backtick use, and a few minor punctuation things to try
to better match the diagnostic style.
This also adds `Error` matchers so that the changes to command line
testing are clearer.
