SAFEGUARD: Check upstream definition string

Obviously John S. Stefano ran into a *cryptic* problem because of a given malformed spooler/upstream definition string. This introduces a runtime check in `cvmfs_server` catching such mistakes with a descriptive error message instead of an assert-crash in the spooler.

Note: this also contains a fine-tuning in testcase 023 which I apparently checked into my local master by accident.