Mentors fails to accept uploads signed with gpg v1

Mentors failed to import the changes powercap_0.3.0-1_source.changes (available in ~lyknode/tmp/) with the following error from the importer:

Sep 26 16:19:41 wv-debian-mentors1 celery[14097]: Failed to import powercap: Changes is invalid                                                                                  
Sep 26 16:19:41 wv-debian-mentors1 celery[14097]: Unknown GPG error. Output was: ['[GNUPG:] NEWSIG', 'gpg: Signature made Sat 26 Sep 2020 04:15:30 PM UTC', 'gpg:                using RSA key EC7F00C2E837EDBB', '[GNUPG:] ERRSIG EC7F00C2E837EDBB 1 2 01 1601136930 9 -', '[GNUPG:] NO_PUBKEY EC7F00C2E837EDBB', "gpg: Can't check signature: No public key"]    

The signature is actually valid and the key match the signature in user's profile.

The issues comes from the output of gpg --verify unable to produce a full fingerprint for the signing key. See the output of powercap versus another package:

$ gpg --verify --status-fd 1 powercap_0.3.0-1_source.changes
[GNUPG:] NEWSIG
gpg: Signature made Sat 26 Sep 2020 06:15:30 PM CEST
gpg:                using RSA key 0xEC7F00C2E837EDBB
[GNUPG:] ERRSIG EC7F00C2E837EDBB 1 2 01 1601136930 9 -
[GNUPG:] NO_PUBKEY EC7F00C2E837EDBB
gpg: Can't check signature: No public key

$ gpg --verify --status-fd 1 fonts-cascadia-code_2009.14-1_amd64.changes
[GNUPG:] NEWSIG
gpg: Signature made Mon 21 Sep 2020 10:17:58 AM CEST
gpg:                using RSA key B60A1BF363DC1319FF0A8E89116852BCDF7515C0
[GNUPG:] ERRSIG 116852BCDF7515C0 1 8 01 1600676278 9 B60A1BF363DC1319FF0A8E89116852BCDF7515C0
[GNUPG:] NO_PUBKEY 116852BCDF7515C0
gpg: Can't check signature: No public key

The problem most likely originate from using a system with gpg v1 to sign the upload. Gpg v1 format must not contain the full fingerprint.

While gpg1 is kind of old, mentors should still accept uploads signed with it. Mentors should fallback to use the first field of ERRSIG when the last equals -.

/cc @ckimes