Add mock tests for various server errors

[jfear]

Need to create a set of mock tests for the various server errors that Chris and I have been running into.

• MongoDB Connection Refused [Errno 61] #13 MongoDB connection refused
• Index Run Invalid #17 problem with not capturing ERROR tag in XML
• URLError End Of File Error #25 URL open EOF error
• MongoDB Server Timeout [Errno 111] #26 MongoDB connectivity issues
• Connection reset by peer [Errno 104] #27 Connection reset by SRA
• URLError timeout [Errno 60] #28 SRA timeout error

[jfear]

Started a project to monitor the progress of these issues.

[Jiahe-Sun]

I have same error!