Improve error messages for bootloader build errors

[theotherjimmy]

The error messages related to missing or broken device names in the
targets.json file were bad/just python tracebacks. This PR changes that
behavior, by having the Config object raise ConfigExceptions for those
mis-configurations.

Resolves #4057

Testing

• Normal /morph test
• Manual exercise of each corner case and verification that the
  messages seen are good.

Manual testing

• mbed_app.json

{
    "target_overrides":{
        "NCS36510":{
            "target.restrict_size": "0x40000"
        }
    }
}

• mbed compile -m NCS36510

[ERROR] Bootloader not supported on this target.

• add "bootloader_supported": true to targets.json
• mbed compile -m NCS36510

[ERROR] Bootloader not supported on this target: targets.json `device_name` not specified.

• add "device_name": "NCS36510" to targets.json
• mbed compile -m NCS36510

[ERROR] Bootloader not supported on this target: targets.json `device_name` not found in arm_pack_manager index.

[theotherjimmy]

/morph test

[sg-]

Need to change the base branch to mbed-os-workshop when CI completes

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 1767

All builds and test passed!

[0xc0170]

/morph test

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 1781

All builds and test passed!

[theotherjimmy]

@sg- Why did you want this retargeted? Are we planning on submitting the workshop branch as a PR after the workshop?

[0xc0170]

@sg- Why did you want this retargeted? Are we planning on submitting the workshop branch as a PR after the workshop?

yes, we are.

[theotherjimmy]

If we are merging the workshop branch into master, should that have been a rebase?

[0xc0170]

If we are merging the workshop branch into master, should that have been a rebase?

yes, would have been easier. we will re-organize the branch a bit (not everything will go to master most likely) . There are some patches that will require further work (thus we are using a branch).

[theotherjimmy]

Thanks for the clarification.