It seems kdenlive does not have an appropriate fallback when ffmpeg is not installed at first run. The setting key `ffmpegpath` does not seem to resolve to an installed ffmpeg instance even after a full close and start of the application.

This is present in (at least) the extract zone job.
 - src/project/jobs/proxyclipjob.cpp



Reproducible: Always

Steps to Reproduce:
(Reproducibility: tricky to reproduce)

These steps assumes the developer does not want to remove ffmpeg to test, it gives out the same result, but it has not been verified that it is the same actual issue.

0. Remove the ffpmeg key from your ~/.config/kdenliverc file. Removing it from the settings panel *does not have the same effect*.

1. Add a clip 
2. Select an in and out point (optional?)
3. Right click the preview and select Extract Zone
4. Accept the options
5. Observe the warning label in the Project Bin (Failed to create file.)
6. Observe the stdout(err?) warning. (QIODevice::read (QProcess): device not open)


Actual Results:  
The process could not be started.

Expected Results:  
Kdenlive should

A. When ffmpeg is not executable:
1. inform the user that ffmpeg could not be found, or try some known sane defaults (/usr/bin/env ffmpeg ?)

B. The error output should be made available to the user, but not necessarily all shown by default.

Everytime:
1. Print the whole command line it tried to execute from QProcess,
2. if the executable process is an empty string (not verified), print some kind of warning? Use some kind of default?

The severity is kind of "Minor", but actually "Grave" for a user, even a power user, since the error is actually opaque, and the dependency is actually present.

(This was reported on behalf of another user, but verified and reproduced by me.)