DFReader: Fix some missing bitmask info in dump verbose

[shancock884]

Following #937, I noticed a few things with bitmask displays that wasn't quite behaving, which this PR aims to fix:

1/ The bits of ArmChecks are not being displayed when doing dump --verbose ARM
This was because these bits have no description, and the except KeyError does not catch this.
Fixed by using if hasattr(bit, 'description') instead.

2/ The last bit was always missing (e.g. CHANGED bit on POWR.Flags)
This was because for i in range(bit_offset) needs a +1.
Actually I realised this should rather be using highest_bit+1 in case more bits are set than defined.

3/ If any bit is missing a definition from the XML, then this and subsequent bit are not shown
This was because bang variable was being used undefined in this case.
Fixed by defining bang earlier, so it is available in either case.

I have also aligned the "!" to be spaced out consistently with how TLOGs and MavProxy show these things.

Tested with a number of examples:
ARM now showing bits:

2024-05-27 11:35:58.749: ARM
    TimeUS: 190359078 µs
    ArmState: 0
    ArmChecks: 0
      ! ARMING_CHECK_ALL
      ! ARMING_CHECK_BARO
      ! ARMING_CHECK_COMPASS
      ! ARMING_CHECK_GPS
...

POWR now showing last bit:

2024-05-27 11:36:13.747: POWR
    TimeUS: 205357102 µs
    Vcc: qnan V
    VServo: qnan V
    Flags: 0
      ! BRICK_VALID (main brick power supply valid)
      ! SERVO_VALID (main servo power supply valid for FMU)
      ! USB_CONNECTED (USB power is connected)
      ! PERIPH_OVERCURRENT (peripheral supply is in over-current state)
      ! PERIPH_HIPOWER_OVERCURRENT (hi-power peripheral supply is in over-current state)
      ! CHANGED (Power status has changed since boot)
...

And by removing the bits except USING_SIGNING from the MAV definition in my local .pymavlink/LogMessages/Plane.xml file:

2024-05-27 11:36:13.274: MAV
    TimeUS: 204884099 µs
    chan: 1 instance
    txp: 15274
    rxp: 624
    rxdp: 0
    flags: 6
      ! USING_SIGNING
        UNKNOWN_BIT1
        UNKNOWN_BIT2
    ss: 0 ms
    tf: 0

[peterbarker]

Merged, thanks!