Fix #1159 (Taurus GUI blocks on DS errors)

When polling various attributes of a Tango device asynchronously (as is done, e.g. by the taurus polling timer), the TaurusDevice.__pollreply() may raise an unhandled exception while calling Tango's read_attributes_reply().

This unhandled exception results in no events being passed to the listener widgets (not even error type events), thus blocking the initialization of some widgets. Fix it by handling the exception as any other read error.

A unit test is provided to catch this problem.

Fixes #1159 (closed)