Print error messages to stderr

[redneb]

Currently ect sends all of its output to stdout, including error messages, e.g.

$ ect doesnotexist.png 2>/dev/null 
doesnotexist.png: bad file
No compatible files found

It would be nice if it sent error messages to `stderr following the common convention in unix.

[fhanau]

I'll keep that in mind, but as this would require a lot of work and there is no stdin/stdout IO support, this is low priority.