Fix some reporting of errors in the GHC API: errors during the
downsweep were thrown as exceptions; now they're reported via the
(Messages->IO ()) callback in the same way as other errors.

getModuleInfo no longer prints anything on stdout.  It does ignore
error messages and return Nothing, however - we should fix this and
return the error messages at some point.

The ErrMsg type can now be thrown as an exception.  This can be a
convenient alternative if collecting multiple error messages isn't
required.  We do this in the downsweep now.