ProcessRunner does not detect launch errors

[aleks-f]

Describe the bug
Launch thread silently ignores any errors or exceptions.

To Reproduce
Launch a non-existing executable.

EDIt: there seems to be no reliable portable way to detect what exactly happens with the launched process (not even when the executable does not exist). Seems best to add the executable file existence check which includes cwd and environment path, and not even try to launch a non-existent executable.