After sending reboot-to-bootloader, the PX4 USB CDC node briefly
disappears while the bootloader re-enumerates. Reopening the serial
port can land on a half-broken descriptor and the next tcdrain()
raises termios.error (5, 'Input/output error'). That bare OSError
escaped every retry layer and crashed the uploader, even though a
manual re-run would succeed once enumeration settled.
Convert OSError/SerialException from flush() and reset_buffers() into
the module's ConnectionError, matching how send()/recv() already
behave, and let the identify retry loops in _try_identify also catch
ConnectionError so a single transient I/O hiccup doesn't abort the
upload.
Signed-off-by: Jacob Dahl <dahl.jakejacob@gmail.com>
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