Stop Greentea tests if a fatal error occurs

[multiplemonomials]

Summary of changes

Quick PR to improve quality of life for us devs! Until now, if a Greentea test obviously died (HardFault or assertion failure), the test runner would just keep on truckin', waiting for the timeout to expire before failing the test. Since some tests (esp. network ones) can have timeouts up to 20 minutes, this could result in a lot of wasted time!

With this PR, the mbed_error() handler will now output a Greentea KV pair that tells the test runner "hey, this test crashed", so it can immediately end the test.

Impact of changes

Should save some dev time during testing!

Migration actions required

Documentation

None

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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