Introduce specialized null component exception
Improve injection point toString methods

This tries to make the injection point toString methods map more directly to Java source.
Provide more descriptive injection exceptions
Add missing license header
Merge branch 'feature/elehack-error-reporting'

Conflicts:
	src/main/java/org/grouplens/grapht/InjectionException.java
	src/main/java/org/grouplens/grapht/reflect/internal/InjectionProviderImpl.java