Report solver name in unsupported-topology error message

[JoelPasvolsky]

Current Problem If the implicitly selected solver is not a QPU for DWaveCliqueSampler(), the current error message,

>>> sampler = DWaveCliqueSampler()
---------------------------------------------------------------------------
KeyError
KeyError: 'topology'
...
ValueError: given sampler has unknown topology format

could be more helpful, if it showed users which solver was implicitly selected.

Proposed Solution Also spit out selected solver's name.

Additional context The current error message is okay, this is just a proposed improvement.

[arcondello]

Should be closed by https://github.com/dwavesystems/dwave-system/pull/317 in the sense that it should only pick up QPUs.