Throw a better exception when factory not found

Previously, if a script engine is requested and not found, the
findFactory method would return null, which would then get handed to the
other constructor, which would then throw NullPointerException with no
explanation of what the problem is.

Author: ctrueden

src/main/java/org/scijava/script/AdaptedScriptLanguage.java

@@ -148,7 +148,7 @@ private static ScriptEngineFactory findFactory(final String factoryName) {
 				if (factoryName.equals(name)) return factory;
 			}
 		}
-		return null;
+		throw new IllegalArgumentException("No such script engine: " + factoryName);
 	}
 
 }