Add a new exception, InconsistencyError

With 'headnode_delete_hnic', we have the first API call where two of the
arguments can conflict with each other: the explicitly provided headnode might
not be the headnode that the given hnic is attached to.  In this situation, I
think we should raise this exception.

(If we eventually move to having hnic names be unique only per-headnode, then
this error will no longer be possible on this API call, but it will likely be
possible elsewhere, and is generally a meaningful error message to give.)
Add parameter 'nodename' to 'headnode_delete_hnic'
Add two more tests to headnode_delete_hnic

These exercise the InconsistencyError that can now be raised.