TF-M post build hooks produce opaque error messages

[rwalton-arm]

Description of defect

The error message produced after failing to sign the binary for any reason is:

Traceback (most recent call last):
      File "/Users/user/Code/mbed-os-example-blinky/mbed-os/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 197, in <module>
        sign_and_merge_tfm_bin(args.tfm_target, args.target_path, args.non_secure_bin, args.secure_bin)
      File "/Users/user/Code/mbed-os-example-blinky/mbed-os/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 80, in sign_and_merge_tfm_bin
        raise Exception("Unable to sign " + target_name +
    Exception: Unable to sign musca_b1 secure binary, Error code: 1

This is not a good error message as no reason is provided for the failure. Root-causing the problem based on this output is more difficult than it should be.

Target(s) affected by this defect ?

All

Toolchain(s) (name and version) displaying this defect ?

All

What version of Mbed-os are you using (tag or sha) ?

master

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-cli 1 or mbed-cli 2

How is this defect reproduced ?

One way to produce this is to forget to install mbed-os's python requirements.

[Patater]

Should be fixed by #14783 which adds better error messages