improve error handling on getoutput

a spurios "failed to retrieve file" is generated when gfal_copy contains a line with the `error` string
https://github.com/dmwm/CRABClient/blob/d4b4151f668ba23cb069569e9613c83776630f6b/src/python/CRABClient/Commands/remote_copy.py#L356
even if the message was harmless and transfer worked OK. E.g.
```
TLS: Unable to create TLS context; invalid private key.
TLS: 47882433451776:error:0B080074:x509 certificate routines:X509_check_private_key:key values mismatch:crypto/x509/x509_cmp.c:303:
```

see https://github.com/dmwm/CRABServer/issues/8357

It is better to rely on gfal-copy exit code first. And only parse stderr in case of failure so somehow translate known messages into more clear error categories.

relevant code is in
https://github.com/dmwm/CRABClient/blob/d4b4151f668ba23cb069569e9613c83776630f6b/src/python/CRABClient/Commands/remote_copy.py#L295-L301
https://github.com/dmwm/CRABClient/blob/d4b4151f668ba23cb069569e9613c83776630f6b/src/python/CRABClient/Commands/remote_copy.py#L333