Notify on configuration exception rather than breaking build

[madchutney]

Summary of changes

Reimplementation of changes to tools/toolchains/mbed_toolchain.py made in #12059. This change allows older versions of Mbed OS to be build in the online compiler but also adds notifications of configuration errors.

See https://jira.arm.com/browse/IOTBTOOL-515 for further details.

Impact of changes

Keeps backward compatibility but does not raise an error on configuration errors.

Migration actions required

None

Documentation

None

---

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)

---

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

---

Reviewers

---

[adbridge]

AFAICT looks ok to me

[adbridge]

@JammuKekkonen could you please review

[adbridge]

For expediency I'm going to run this through the CI will we await other reviews

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[JammuKekkonen]

Should this go to 5.15 branch only?

[madchutney]

Should this go to 5.15 branch only?

If so wouldn't next release break the online compiler backwards compatibility again?

[adbridge]

I think we have to take this for this release and then work out what we do going forward

[0xc0170]

👍 for fixing this