The user-supplied value for timecode_rate in drawtext is rounded
to nearest integer. So, a supplied value of 0.49 or lower is rounded to 0.
This throws a misleading error message which says "Timecode frame rate must be
specified". Changed message to account for values under one.

Also noted supported framerates for drop TC.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>