This allows for a color to be set when the wallpaper does not fill the
entire output. If specified, the fallback color is also used when the
image path is inaccessible.
- fixes a double-free error when access() failed.
- refactor code to make memory managment (alloc/free) more straightforward
- do not bring the temporary wordexp_t struct around
- do not postpone errors handling
if src is NULL due to a previous error we cannot use it in the command
result string.
Moreover if `src` points to `p.we_wordv[0]` we cannot use it after
`wordfree(&p)` in the command result string.
Bonus feature: If there was an error accessing the file, the string
rapresentation of the error is now included in the command result
string.
