When failing to symbolically linking files, display the names of the
files that could not be linked. This makes it easier to troubleshoot the
error without having to manually figure out which files failed to link.
Linker now reports an error when links are configured to point to
nonexistent targets. This fixes the old behavior where Linker silently
created invalid links.
According to PEP 394, `python` should only be used in the shebang line
for scripts that are source compatible with both Python 2 and Python 3.
In previous versions of Dotbot, on certain systems where `python`
referred to `python3`, running Dotbot would throw an exception due to a
SyntaxError. This can be fixed by making Dotbot source compatible with
both Python 2 and Python 3.
