Quit if no file found at all and tell when using last profile data

mprof

@@ -431,6 +431,7 @@ def plot_action():
                   "mprofile_*.dat files, generated by the "
                   "'mprof run' command.")
             sys.exit(-1)
+        print("Using last profile data.")
         filenames = [profiles[-1]]
     else:
         filenames = []
@@ -445,6 +446,9 @@ def plot_action():
                         filenames.append(profiles[n])
                 except ValueError:
                     print("Input file not found: " + arg)
+    if not len(filenames):
+        print("No files found from given input.")
+        sys.exit(-1)
 
     pl.figure(figsize=(14, 6), dpi=90)
     if len(filenames) > 1 or options.no_timestamps: