LWIP: propagate the apimsg->err out of netconn_apimsg()

[michalpasztamobica]

Summary of changes

Fixes #12503.
The error code is now propagated correctly out of netconn_apimsg(). So far ERR_OK was always returned. Compare to the original LWIP's code a few lines below, where the fn() return value isn't checked either, so clearly there must be an assumption that the function will set the error code appropriately. I think we should follow the LWIP's way.
See also the issue and the discussion in there for more details.

Impact of changes

Error code propagation improvement.

Migration actions required

None

Documentation

None

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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
@SeppoTakalo
@kjbracey-arm
@JojoS62

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[ciarmcom]

@michalpasztamobica, thank you for your changes.
@AnttiKauppila @SeppoTakalo @kjbracey-arm @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts

[mergify]

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