XML parse errors parsing metadata payload

[sdm7g]

I'm using oaipmh via oai-harvest.
When the program hits an exception from a parse error in the metadata payload, it quits and stops harvesting. ( Typically not a problem with simple oai_dc metadata, but oai_ead has a lot more to get correct, and I'm seeing this frequently on several ArchivesSpace OAI feeds. )
I have found that back patching the Client.parse method to create an XMLParser with recover=True manages to finish harvesting the entire feed.
etree.XML(xml, etree.XMLParser(recover=True))

You might consider adding that as an option on creating Client, or as a subclass.

[sdm7g]

Withdrew #38 and reimplemented improved version #39
minimal changes. Default behavior is the same as before. Adds optional recover=True|False arg.
I discovered lxml.etree has it's own code to manage how errors are logged, so additional error logging code was unnecessary.