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.
