[dfu] print exception
[supervision] allow launching of external tools with absolute path
[rotorcraft] use horizontal guidance ref by default

Also use the horizontal guidance reference for HOVER mode.
Using the reference can be temporarly disabled via settings.
The reference is then still computed to stay consistent, but not used.
[nps] don't bypass ahrs by default

By default use the result of the normal ahrs estimation,
if you want to bypass that and use the attitude from NPS define NPS_BYPASS_AHRS to TRUE.
[rotorcraft][messages] GUIDANCE_H_INT message
[airframes] update GUIDANCE_H gains for quadrotor example
[stm32] Fixes for USART interrupt handler.

The interrupt handler occasionally hangs in case of noise on the line.
Fixes were two:
1) Disable IDLE line interrupt, disable LIN break interrupt in the port settings.
   (neither of them should be used for asynchronous single buffer communication).
2) In interrupt handler routine, explicitly check for the error flags, only if
   no errors occur check the received byte. Otherwise clear the USART_DR and
   log the error.

Added USART_ERRORS messages to monitor problems on USART lines.

Closes #418
[rotorcraft] silence warning in guidance
[rotorcraft] alternate buses when sending I2C_ERRORS message
Merge remote-tracking branch 'upstream/master'