Warn on missing state transition

[wtgee]

Logs a warning if not valid transition exists between state and next_state. Still returns parking.

Removes unnecessary variable creation.

Related Issue

Closes #1110

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@b820859). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #1131   +/-   ##
==========================================
  Coverage           ?   83.13%           
==========================================
  Files              ?       86           
  Lines              ?     7390           
  Branches           ?      627           
==========================================
  Hits               ?     6144           
  Misses             ?     1074           
  Partials           ?      172           

Impacted Files	Coverage Δ
src/panoptes/pocs/__init__.py	100.00% <ø> (ø)
src/panoptes/pocs/base.py	100.00% <ø> (ø)
src/panoptes/pocs/camera/__init__.py	70.78% <ø> (ø)
src/panoptes/pocs/camera/camera.py	84.77% <ø> (ø)
src/panoptes/pocs/camera/gphoto/__init__.py	100.00% <ø> (ø)
src/panoptes/pocs/camera/gphoto/base.py	100.00% <ø> (ø)
src/panoptes/pocs/camera/sdk.py	88.63% <ø> (ø)
src/panoptes/pocs/camera/simulator/ccd.py	94.52% <ø> (ø)
src/panoptes/pocs/camera/simulator/dslr.py	95.58% <ø> (ø)
src/panoptes/pocs/core.py	92.64% <ø> (ø)
... and 2 more

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