Fix error message for find_program if program cannot be found

Currently this raises an exception because exename is undefined.

mesonbuild/interpreter.py

@@ -2017,7 +2017,7 @@ class Interpreter(InterpreterBase):
         if progobj is None:
             progobj = self.program_from_system(args)
         if required and (progobj is None or not progobj.found()):
-            raise InvalidArguments('Program "%s" not found or not executable' % exename)
+            raise InvalidArguments('Program "%s" not found or not executable' % args[0])
         if progobj is None:
             return ExternalProgramHolder(dependencies.ExternalProgram('nonexistingprogram'))
         return progobj