editor: better error message on paste failure.

Apparently I was incredibly lazy at some point in the past and forgot
to say what the problem was when the paste data couldn't be matched
against the regex.

editor

@@ -175,7 +175,8 @@ class EditorGui:
         for i in range(YSIZE):
             m = pat.search(s)
             if m is None:
-                print("gronk")
+                print("Unable to interpret selection data {!r} as a "
+                      "Bedstead glyph description".format(s))
                 return
             bitmap.append(int(m.group(0), 8) & ((1 << XSIZE) - 1))
             s = s[m.end(0):]