Modify the install phase to detect when nothing has been installed, and error
if this happens. This is preferable to continuing, and allowing the next phase
to fail.
Also, when nothing can be found to be installed, print out each file that was
considered, along with the regular expressions that were used to include and
exclude it.
* gnu/build/emacs-build-system.scm (install-file?): Add additional error
checking and logging.
