Better error message if 'remove-project' refers to non-existent project

If a local manifest includes a 'remove-project' element that refers to
a project that does not exist in the manifest, the error message is a
bit cryptic.

Change the error message to make it clearer what is wrong.

Change-Id: I0b1043aaec87893c3128211d3a9ab2db6d600755