No longer throw NPE when profile property is undefined

If a profile included a property such as ${appName}, and that property
was not defined somewhere (usually in the <properties> element in the
pom), BuildMojo would throw a NPE. We now throw a MojoExecutionException
with a message saying which property was undefined.
Merge pull request #10 from spotify/rculbertson/undefined-property-npe

No longer throw NPE when profile property is undefined