ARMmbed / mbed-os / 21964

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Tools: Don't traceback on missing linker script

### Description

The `mbed compile` would traceback when no linker script is found.
This PR changes that behavior to make that into a NotSupportedException,
which has decent user behavior.

Fixes #7723

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change

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