I tried to use a map of libraries with references from subprojects.
I spent few hours on diving into logfiles, adding messages, changing
buildscripts line by line, before I discovered my typo in dependency
references.

The output from gradle was:
---------<snip>----------------
FAILURE: Build failed with an exception.

• Where:
  Build file '.../build.gradle' line: 2

• What went wrong:
  A problem occurred evaluating project ':library'.
  Cause: The dependency notation: null is invalid.
  --------<snap>----------------

and the logs (-d option) did not gave any further hint.
The referred errorline was the start of dependencies.

Don't know, whether it is possible, but I would appreciate an error-message
telling about which key not found in the map or something similar. It's
completely ok, have that information at debug-level only.