Export variable substitution, add more tests and support an error handler

Having variable_substitution as a public member can be useful for other modules who want to be able to replace variables without having to use the file related subroutines.

For example, in GNOME packages the debian/control file is generated from a debian/control.in and so it would be nice to use it to replace some variables, without breaking the various ${a:b} substvar (see gnome-team/gnome-pkg-tools!1).

As per this, we would also need to be able to handle substitution errors gracefully, so added this together with tests.