Failure when there is no master pseudoclock is ungraceful

[philipstarkey]

Original report (archived issue) by Chris Billington (Bitbucket: cbillington, GitHub: chrisjbillington).

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If you don't define a master pseudoclock (or any devices), you get this obtuse thing:

#!python

Traceback (most recent call last):
  File "/home/bilbo/Desktop/user_scripts/labscriptlib/spinor/__init__.py", line 8, in <module>
    start()
  File "/home/bilbo/bitbucket_clones/labscript/labscript.py", line 1593, in start
    master_pseudoclock, = [pseudoclock for pseudoclock in all_pseudoclocks if pseudoclock.is_master_pseudoclock]
ValueError: need more than 0 values to unpack

This is pretty simple, let's put a better error message there if stop() gets called and there is no master pseudoclock

[philipstarkey]

Original comment by Chris Billington (Bitbucket: cbillington, GitHub: chrisjbillington).

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• changed state from "new" to "resolved"

Resolves issue #14 and issue #12

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