lib/gpg: Add more specific OstreeGpgError codes

Currently `ostree_gpg_verify_result_require_valid_signature` always
returns an error that the key used for the signature is missing from the
keyring. However, all that's been determined is that there are no valid
signatures. The error could also be from an expired signature, an
expired key, a revoked key or an invalid signature.

Provide values for these missing errors and return them from
`ostree_gpg_verify_result_require_valid_signature`. The description of
each result is appended to the error message, but since the result can
contain more than one signature but only a single error can be returned,
the status of the last signature is used for the error code. See the
comment for rationale.

Related: flatpak/flatpak#1450

tests/test-pull-summary-sigs.sh

@@ -189,7 +189,7 @@ cp ${test_tmpdir}/ostree-srv/gnomerepo/summary.sig{.2,}
 if ${OSTREE} --repo=repo pull origin main 2>err.txt; then
     assert_not_reached "Successful pull with old summary"
 fi
-assert_file_has_content err.txt "none are in trusted keyring"
+assert_file_has_content err.txt "BAD signature"
 assert_has_file repo/tmp/cache/summaries/origin
 assert_has_file repo/tmp/cache/summaries/origin.sig
 cmp repo/tmp/cache/summaries/origin ${test_tmpdir}/ostree-srv/gnomerepo/summary.1 >&2

tests/test-remote-gpg-import.sh

@@ -163,7 +163,7 @@ ${OSTREE} remote add --set=gpgkeypath=${test_tmpdir}/gpghome/key1.asc,${test_tmp
 if ${OSTREE} pull R8:main 2>err.txt; then
     assert_not_reached "Unexpectedly succeeded at pulling with different key"
 fi
-assert_file_has_content err.txt "GPG signatures found, but none are in trusted keyring"
+assert_file_has_content err.txt "public key not found"
 
 # Test gpgkeypath success with directory containing a valid key
 ${OSTREE} remote add --set=gpgkeypath=${test_tmpdir}/gpghome/ R9 $(cat httpd-address)/ostree/gnomerepo
@@ -243,7 +243,7 @@ ${OSTREE} remote add --set=gpgkeypath=${test_tmpdir}/gpghome/key2.asc R6 $(cat h
 if ${OSTREE} pull R6:main 2>err.txt; then
     assert_not_reached "Unexpectedly succeeded at pulling with different key"
 fi
-assert_file_has_content err.txt "GPG signatures found, but none are in trusted keyring"
+assert_file_has_content err.txt "public key not found"
 
 echo "ok"
 
@@ -269,7 +269,7 @@ newrev=$(${CMD_PREFIX} ostree --repo=${test_tmpdir}/ostree-srv/gnomerepo rev-par
 if ${OSTREE} pull --require-static-deltas R1:main 2>err.txt; then
     assert_not_reached "Unexpectedly succeeded at pulling commit signed with untrusted key"
 fi
-assert_file_has_content err.txt "GPG signatures found, but none are in trusted keyring"
+assert_file_has_content err.txt "public key not found"
 
 echo "ok gpg untrusted signed commit for delta upgrades"