Don't warn if target reports no threads

[Vladimir Prus]

In the case when stub refuses to understand all modern thread-listing packets,
GDB falls back to qL packet, if if stub refuses to handle that either, 
reports:

	RMT ERROR : failed to get remote thread list.

The further operation continues normally, and qL is really optional packet, so
there's no point scaring a user like that. Is the patch below OK?

- Volodya

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