Replace with weak mbed_error_hook implementation (deprecate the hook function)

[DBS06]

Summary of changes

mbed_set_error_hook() can only be called inside the application. Therefore it is not possible in the application to catch errors before calling mbed_set_error_hook().
I thought it would be simpler to create a mbed_error_hook() function which is weak and the application can catch errors at any time.

Impact of changes

Replace mbed_set_error_hook() with an application specific mbed_error_hook() implementation.

Migration actions required

Documentation

---

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)

---

Test results

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

---

Reviewers

---

[kjbracey]

Removing an existing API with no prior warning isn't ideal.

Conceivably to retain compatibility you could have "call the registered handler" be the default weak implementation, and have the ability to override that.

Although that makes it more complex, rather than simpler.

Anyone else have thoughts?

[DBS06]

Maybe we could set this in mbed-os 5.15.x to deprecated and change it in mbed-os 6.x.x?

[ciarmcom]

@DBS06, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[evedon]

I like the idea of using a weak mbed_error_hook implementation but we should only deprecate mbed_set_error_hook() in this PR. Removal should come in a subsequent Mbed 6 release.

[DBS06]

Is it okay to do it like in my last commit?

[DBS06]

How should I interpret astyle error and how do I fix it?

[0xc0170]

Travis astyle job prints the failures, see https://travis-ci.org/github/ARMmbed/mbed-os/jobs/658578979

There is spaces left in the code (check for empty spaces after code or comment), all 3 offenders in the code style.

[0xc0170]

In this case, weakly linked is better than a hook.

Although working recently on some CMake where you get an issue with weakly defined symbols in libraries is not ideal world 🙄 but that is not related here, so all fine.

Pull request has been modified.

[adbridge]

@evedon @kjbracey-arm what are your thoughts on this now ?