Fail with descriptive errors when coordinator is disconnected

[puddly]

AttributeError: 'NoneType' object has no attribute 'request_callback_rsp' makes it sound like there is a library bug.

[codecov-commenter]

Codecov Report

Base: 98.52% // Head: 98.52% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (adf6df7) compared to base (36c5d09).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #203   +/-   ##
=======================================
  Coverage   98.52%   98.52%           
=======================================
  Files          43       43           
  Lines        3799     3803    +4     
=======================================
+ Hits         3743     3747    +4     
  Misses         56       56           

Impacted Files	Coverage Δ
zigpy_znp/api.py	96.95% <100.00%> (+0.24%)	⬆️
zigpy_znp/zigbee/application.py	94.80% <100.00%> (-0.24%)	⬇️

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