J-Link could start core error

[flit]

Sometimes the J-Link driver will report that it could not start the CPU core when pyocd disconnects.

Example error from running test/basic_test.py on a Nordic nRF5340-DK using the J-Link OB:

0011878:ERROR:jlink:Could not start CPU core. (ErrorCode: -1)

It's not clear what the cause of this is, whether it's a bug in pyocd (likely) or something in the target state.