better error handling by OAIPMHGenerator

[markmatney]

The code currently has no checks for thrown exceptions.

[hariharshankar]

Could you please elaborate on this?

[markmatney]

My generator doesn't do error handling very well, in that for example, it's possible that the call to requests.get could return a 404, which should be checked (in which case there would be no XML content, and trying to parse the response body with BeautifulSoup may throw an error).