#157 No error provided if a user renames a build file

[Remy Chi Jian Suen]

If a user renames a build file without altering the project's properties, the builder will try to invoke a build (in reaction to the resource change), and subsequently fail.

NAnt will output something like the following:
Could not find file "/home/rcjsuen/runtime-New_configuration/b/build.xml".

The user knows that the file does not exist (since they renamed it themselves) but it may not be immediately obvious as to what they need to do to correct Emonic's builder to point at the renamed file.