tools/mavgen.py can silently fail if an enum definition is missing

[nexton-winjeel]

Running mavgen.py in our CI tool can report success even if the generation fails. In this particular case, we had a message defined, but the associated enum was missing.

• Using master at: 19e385c
• Using Python v3.8

(venv) nexton@laptop:~/Repositories/mavlink$ python --version
Python 3.8.10
(venv) nexton@laptop:~/Repositories/mavlink$ ./pymavlink/tools/mavgen.py --output ./build --lang C --wire-protocol 2.0 message_definitions/v1.0/all.xml
Validating message_definitions/v1.0/all.xml
Parsing message_definitions/v1.0/all.xml
[...]
Parsing /home/nexton/Repositories/mavlink/message_definitions/v1.0/cubepilot.xml
Validating /home/nexton/Repositories/mavlink/message_definitions/v1.0/csAirLink.xml
Parsing /home/nexton/Repositories/mavlink/message_definitions/v1.0/csAirLink.xml
Merged enum MAV_CMD
[...]
Merged enum MAV_CMD
Enum ESC_FAILURE_FLAGS in ESC_INFO.failure_flags does not exist
(venv) nexton@laptop:~/Repositories/mavlink$ echo $?
0

[nexton-winjeel]

Fixed in #863.