bug#54210: 27.1; appt-add checks for bad data only at end

[Francesco Potortì]

appt-add asks the user for three parameters:
- time (a time string)
- message (a string)
- minutes (a number)

if the time string is badly formatted (for example if it is "8" rather than 
"8.00") appt-add barfs.  That's ok, but it barfs only after all three 
parameters have been read.  It should barf immediately after submitting the 
wrong parameter.

Maybe I'll find the time to look at it and propose a patch, but since I don't 
know, I prefer reporting it now.