EZSP startup reset corner cases

[puddly]

There are a few corner cases left with EZSP startup:

1. EZSP remaining open if config writing fails (e.g. disconnect)
2. Disconnect during non-startup reset

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (251a1de) 99.79% compared to head (dec3e65) 99.79%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #587   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files          68       68           
  Lines        4868     4869    +1     
=======================================
+ Hits         4858     4859    +1     
  Misses         10       10           

Files	Coverage Δ
bellows/uart.py	100.00% <100.00%> (ø)
bellows/zigbee/application.py	99.62% <100.00%> (ø)

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