mbed compile does not fail when failing to parse mbed_app.json #481
Description
When there is invalid json in the mbed_app.json file, mbed compile will display an error message like e.g
Error parsing './mbed_app.json':
Expecting property name: line 14 column 1 (char 262)
However, it will not cause the command to fail, resulting in a binary that does not contain any of the configuration set in mbed_app.json and the error message to get lost in the output of the compilation. This is a problem that can be easily triggered by e.g improperly removing an entry in the target_overrides resulting in a trailing comma and thus invalid json.
Not failing on parsing the mbed_app.json file is also inconsistent with other errors related to the configuration like e.g overriding an undefined parameter, which causes mbed compile to fail with an error message like
[ERROR] Attempt to override undefined parameter 'target.fubar' in 'application[SOME_TARGET]'