Allow MBED_ASSERT to throw errors in unittests, so we can verify it.

[SeppoTakalo]

Description

Summary of change

GoogleTest allows you to check whether certain function throws errors.
For example:

mbed_assert_throw_errors=true;
ASSERT_ANY_THROW(mbed::HeapBlockDevice one(3050, 100));

or

mbed_assert_throw_errors=true;
ASSERT_NO_THROW(bd.init());

As MBED_ERROR is now only function that can throw errors, there is
no need to check the type of thrown object.

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] 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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Reviewers

@AnttiKauppila

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Release Notes

Summary of changes

Impact of changes

Migration actions required

[mbed-ci]

Test run: SUCCESS

Summary: 1 of 1 test jobs passed
Build number : 1
Build artifacts

[mbed-ci]

Test run: SUCCESS

Summary: 1 of 1 test jobs passed
Build number : 2
Build artifacts

[ciarmcom]

@SeppoTakalo, thank you for your changes.
@AnttiKauppila @ARMmbed/mbed-os-core @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Please review.

I've noticed this one is in #11918 that shall go to 5.15 (in that case, this should be reviewed now).

[SeppoTakalo]

Reviewed and CI passed.

Should be good to go.