GPS PreflightCheck: improve failure reporting
GPS PreflightCheck: improve failure reporting

- previously it was possible to get a Position Control rejected message
  without further advice what was actually wrong. So now we report warnings
  even if gps is not required for arming (which could be annoying too...).
- the GPS failure message was very generic, making it hard to debug the
  cause. Now we check every bit and send an appropriate warning

All strings were checked not to exceed the maximum length of 50 characters.

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