fix harvesting logic of metadata typenames/formats

[tomkralidis]

(originally reported as part of #96 by @rhodges)

• pycsw does CSW harvesting with the following logic:
  • client sends CSW Harvest request asking server A to harvest server B
  • server A sends 1..n GetRecords server B asking for all typenames supported by server A
  • if server B does not support all server A's typenames, an exception is thrown by server B, and subsequently server A back to the client

This logic needs to be updated to safeguard against asking for typenames that a CSW doesn't support.

[tomkralidis]

@rhodges : FYI there was an underlying issue in OWSLib (fixed in geopython/OWSLib@266c3f6 -- note that I haven't bumped OWSLib to PyPI, will do so soon). I've also put some improved error handling in pycsw.

Can you please test this out and let us know if this works for you?

[rhodges]

Tests went smoothly, nice work!