#222 Improve feedback when inexisting filter is specified

[Kalle Richter]

I ran into an issue where I could specify the motion_est filter on the command line although it was not installed/build due to missing --enable-gpl argument for configure. Rather than let the program run and suceed with an unusable result (my example use case is melt file.mp4 -attach motion_est -consumer xml all=1 where split markers are provided in the filter element) it should fail it it encounters a missing filter. The presence of the filter can be validated by the invoker with melt -query "filter" | grep filtername or very similar in another language.

Although the missing filter element in the XML output is a deterministic hint that the filter is missing, the fact that the invocation succeeds is not very intuitive.

Based on https://sourceforge.net/p/mlt/mailman/message/33233377/.