In `cb57e6560fe7767e79b87473a11261fbb82ce84c` the loading of systemd
credentials was refactored. The idea was to only change the accepted IDs
slightly after switching from radicle.xyz to radicle.dev. Unfortunately,
this was done in a sloppy way. The two cases for passphrase and secret
actually differ in ways that was overlooked.
Rectify these mistakes and restore the old behavior:
1. The logic to read the passphrase from the file that contains the
credential is moved out of `load_credential` since it only applies
to the passphrase, not the secret.
2. The warnings printed are now generic, and will not mention the
passphrase in case reading the path for the secret fails.
0a0e70b1d1