Clarify adb bugreport failures on pre-N devices.

The error was meant to imply "without the filename" but (a) that wasn't
spelled out and (b) anyone who did just try the command would probably
be unpleasantly surprised by the massive amount of spam on their
terminal. So give them copy & paste instead.

(I did consider using their supplied filename, but since that's almost
certainly blah.zip, it seemed uncool to silently create a large text
file called "something.zip"!)

Bug: http://b/170225883
Test: untested for lack of a working pre-N device right now
Change-Id: I834939c963ca09927ccd4dc5ed8e88c65455838e

adb/client/bugreport.cpp

@@ -224,7 +224,8 @@ int Bugreport::DoIt(int argc, const char** argv) {
         // 'bugreport' would generate a lot of output the user might not be prepared to handle).
         fprintf(stderr,
                 "Failed to get bugreportz version: 'bugreportz -v' returned '%s' (code %d).\n"
-                "If the device does not run Android 7.0 or above, try 'adb bugreport' instead.\n",
+                "If the device does not run Android 7.0 or above, try this instead:\n"
+                "\tadb bugreport > bugreport.txt\n",
                 bugz_output.c_str(), status);
         return status != 0 ? status : -1;
     }