python: add details on missing library to error message (#9332)

When a library cannot be found, the exception now includes the name
of the library that was requested.

var/spack/repos/builtin/packages/python/package.py

@@ -528,7 +528,7 @@ def libs(self):
                 return LibraryList(os.path.join(frameworkprefix, ldlibrary))
             else:
                 msg = 'Unable to locate {0} libraries in {1}'
-                raise RuntimeError(msg.format(self.name, libdir))
+                raise RuntimeError(msg.format(ldlibrary, libdir))
         else:
             library = self.get_config_var('LIBRARY')
 
@@ -538,7 +538,7 @@ def libs(self):
                 return LibraryList(os.path.join(frameworkprefix, library))
             else:
                 msg = 'Unable to locate {0} libraries in {1}'
-                raise RuntimeError(msg.format(self.name, libdir))
+                raise RuntimeError(msg.format(library, libdir))
 
     @property
     def headers(self):