Improve error message in case sync.{user/password} are incorrect

[LukasKalbertodt]

Fixes #828

Error message before:

▸ Connection to Opencast harvesting API  ✘ error
     ▶▶▶ Error: failed to fetch API version
     Caused by:
      ‣ API returned unexpected HTTP code 401 Unauthorized (for 'https://develop.opencast.org/tobira/version')

After:

 ▸ Connection to Opencast harvesting API  ✘ error
      ▶▶▶ Error: failed to fetch API version
      Caused by:
       ‣ Requesting 'https://develop.opencast.org/tobira/version' with login 'admin:******' returned 401 Unauthorized. Check 'sync.user' and 'sync.password'!

Its not ideal that the URL is in the beginning but I haven't found another good way to phrase that. In either way, it should be clear what the problem is.

[owi92]

Working, and definitely a convenient improvement.