As the get_pixbuf_from_file should retrieve a file's content, it
shouldn't fail silently, if the file isn't there.
For codepaths, which don't use the `icon_path` to search for a given
icon, it gives a clearer error message.
The icon_path has no point for searching an absolute filename. It was
actually meant to avoid the complex lookup algorithm of the icon theme
spec. And the iconname in this case should be actually just the icon's
basename when searching the path.
Concatenating icons like `~/testimg.png` with all icon paths has no
point and also an additional suffix has no point.
Cleans up the clutter in draw and since only a single icon function is
called externally from icon.c it'll make things easier if we ever decide
to switch icon libraries.
Rename both functions to make clear, what the difference between the
previous functions _from_file and _from_path is. Also remove the
superfluous `== NULL` checks as these don't match dunst's current style.
