We did no real error checking to see if a mode is supported when it
 is created.
This patch implements checks for Jean-Marc's rules:
1) A mode must have frames at least 1ms in length (no more than
    1000 per second).
2) A mode must have shorts of at most 3.33 ms (at least 300 per
    second).
It also adds error checking to dump_modes so we report the error
 instead of crashing when we fail to create a mode.