Copter: Give better error in opendroneid build when DID_ENABLE=0.

[shipp02]

Based on my report on discord:

Problem arises when:

• ArduPilot compiled with --enable-opendroneid
• DID_ENABLE set to 0
• init and update functions do not do anything but pre_arm_checks still fail.

Should pre_arm_checks also be skipped or there should be a different error message like DID_ENABLE is not allowed to be 0 when compiled with --enable-opendroneid.

Currently one of the pre_arm_checks will fail but there's no indiciation of what exactly the problem is. A better error message or disabling pre arm checks might save time for users.

This PR adds a better error message to report that DID_ENABLE is not allowed to be 0 when compiled with --enable-opendroneid.

[peterbarker]

autotests need to pass; looks like the new check in here is causing them to fail, hinting to me that it is possible to compile opendroneid in yet have pre-arm-checks pass even with the option off.

[peterbarker]

@shipp02 I've squashed commits together and fixed a whitespace change problem.

Can you state exactly what the prearm-failure you were getting before this PR?

fixed