Improve license err msg (#24117)

* Add to the error message to help determine failure source.

* Break up long line to keep under 80 chars.

Co-authored-by: Rob Groner <rug262@psu.edu>

lib/spack/spack/cmd/license.py

@@ -175,7 +175,8 @@ def wrong_spdx_identifier(line, path):
             if error:
                 return error
 
-    print('{0}: the license does not match the expected format'.format(path))
+    print('{0}: the license header at the top of the file does not match the \
+          expected format'.format(path))
     return GENERAL_MISMATCH