I give you, below, a generic answer (I know little about ffmpeg/avconv).
Usually, a command complains when something goes wrong. A command that exits
without outputting anything is a command that worked. You can check the exit
status of the command too. Right after it terminates (no command in
between), execute in the same terminal:
$ echo $?
0 means a successful termination, whereas a different number means an error.
If that number is in /usr/include/sysexits.h (see the end of the file), the
type of error probably is the one explained in a comment right after the
number. E.g., if you get the exit status 65, you can search the meaning of
it in this way:
$ grep 65 /usr/include/sysexits.h
#define EX_DATAERR 65 /* data format error */
But you can get an exit status that is not in /usr/include/sysexits.h. And
even if it is in /usr/include/sysexits.h, you may get more information about
the actual error for the command you executed (/usr/include/sysexits.h is
generic) by searching its documentation (typically the 'man' or 'info' page).
