Fix 'netsocket: several dynamic allocation results not checked' (#14210)

[chrisswinchatt-arm]

(Copied from #14706)

Summary of changes

This changes adds std::nowthrow and the checks for allocation failure to all places in the netsocket portion that lacked it. This change addresses #14210.

Impact of changes

add_event_listener in NetworkInterface now returns an error if the method fails. Previous attempts to add the event listener would attempt to use an unchecked standard dynamically allocated ns_list_* item.

In other cases, the dynamically allocated items will now be checked, and if unsuccessful, will return after cleaning up any outstanding issues.

TCPSocket::accept will now check that its own internally allocated new TCPSocket call will succeed, and if not, will clean up the stack resources. This should help when memory is low but an incoming connection requests a connection when the TCPSocket is listening.

Migration actions required

As new calls are now handled, code that did not check against this failure may now check for failure and handle it at the application layer.

add_event_listener now returns nsapi_error_t instead of void. The two return values possible are NSAPI_ERROR_OK and NSAPI_ERROR_NO_MEMORY in the case of memory allocation failure.

Documentation

NetworkInterface.h now has changes to the add_event_listener method. This method will now return nsapi_error_t instead of void. The docstring for the interface has been updated.

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)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

Reviewers

[ciarmcom]

@chrisswinchatt-arm, thank you for your changes.
@ARMmbed/mbed-os-connectivity @ARMmbed/mbed-os-maintainers please review.

[ciarmcom]

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

[0xc0170]

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