No warning/error on non-existing modules

[strongly-typed]

You can add arbitrary module lines like <module>This does not exist and is bullshit</module> and there will be no error or warning. This is very unpleasant if modules names in modm changed and you will get no feedback from lbuild that a requested module was not found.

Accidentally reported against wrong project dergraaf/library-builder#31

[salkinium]

Uuups, that's not good. That's missing test coverage, I'll look into it.