workflows: arXiv/DOI matching task improvement

I was testing locally harvesting and stuff and came across some issues:

* Takes the configuration of WORKFLOWS_MATCH_REMOTE_SERVER_URL from
  the global scope.

* Forces lowercase string matching for arXiv prefix stripping.

* Adds some error handling if the remote requests suffers
  connection issues.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>

ping @GiorgosPa