config: Consolidate error reporting.

This modifies the initial configuration error handling logic to match
dcrd.  In particular, the logic is modified to avoid a bunch of
duplicate code and fix some cases that were not logging the errors as
expected.

It accomplishes this by consolidating the error reporting in a single
place in the caller and introducing a custom error type which signals
that the usage message should be suppressed in order to retain the
primary behavior the originally was the reason for the prints being in
the config function to begin with.

Also, the application name is passed into the function now since it is
needed in both places.