Unify implementations of Status and DBException

This is prep work for a larger improvement to our error handling code. This ticket covers:

• Removing location() from status and the _shard member from DBException, so that they logically hold the same data
• Make DBException hold a Status and kill off the ExceptionInfo type
• Make log() << ErrorCodes::SomeError use the name of the error
  rather than the number
• Make argument order consistently code then message
• Flatten class hierarchy to eliminate MsgAssertionException and UserException into AssertionException. All other
  types will remain, as will the various assert macros which will now
  just throw AssertionException. The difference between massert vs uassert will
  now just that massert logs.