lib/gpg: Use nicer helper for gpg error messages

lib/gpg: Use nicer helper for gpg error messages

The vast majority of invocations of `ot_gpgme_error_to_gio_error()` were paired
with `g_prefix_error()`; let's combine them for the same reason we do
`glnx_throw_errno_prefix()`. For the few cases that don't we might as well add
some prefix.

I also changed it to `return FALSE` in prep for more style porting.

Closes: #1135
Approved by: jlebon