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[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #587 +/- ## ======================================= Coverage 99.79% 99.79% ======================================= Files 68 68 Lines 4868 4869 +1 ======================================= + Hits 4858 4859 +1 Misses 10 10 ``` | [Files](https://app.codecov.io/gh/zigpy/bellows/pull/587?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zigpy) | Coverage Δ | | |---|---|---| | [bellows/uart.py](https://app.codecov.io/gh/zigpy/bellows/pull/587?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zigpy#diff-YmVsbG93cy91YXJ0LnB5) | `100.00% <100.00%> (ø)` | | | [bellows/zigbee/application.py](https://app.codecov.io/gh/zigpy/bellows/pull/587?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zigpy#diff-YmVsbG93cy96aWdiZWUvYXBwbGljYXRpb24ucHk=) | `99.62% <100.00%> (ø)` | |

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