AP_GPS: improve reporting of backend inaccurancy

[peterbarker]

I didn't find this debug message particularly informative; add more info.

Old:

calculate_moving_base_yaw:372: Exceeded the permitted error margin 29.73999 > 0.052001
calculate_moving_base_yaw:372: Exceeded the permitted error margin 29.73999 > 0.052001
calculate_moving_base_yaw:372: Exceeded the permitted error margin 29.73999 > 0.052001
calculate_moving_base_yaw:372: Exceeded the permitted error margin 29.73999 > 0.052001

New:

calculate_moving_base_yaw:372: Offset=0.300000 vs reported-distance=0.250000 (max-delta=0.050000)
calculate_moving_base_yaw:372: Offset=0.300000 vs reported-distance=0.250000 (max-delta=0.050000)
calculate_moving_base_yaw:372: Offset=0.300000 vs reported-distance=0.250000 (max-delta=0.050000)
calculate_moving_base_yaw:372: Offset=0.300000 vs reported-distance=0.239990 (max-delta=0.047998)

Only affects things when you've got the compile-time debug option enabled