Improve launch error message.

The previous error message was confusing for new users:

  libguestfs: error: guestfs_mount: call launch() before using this function

The new error message explains the action that the user must take,
especially if they are using guestfish:

  ><fs> mount /dev/sda1 /
  libguestfs: error: guestfs_mount: call launch before using this function
  (in guestfish, don't forget to use the 'run' command)

diff --git a/src/generator.ml b/src/generator.ml
index e3293d1..d3040d5 100755 (executable)
--- a/src/generator.ml
+++ b/src/generator.ml
@@ -3737,7 +3737,7 @@ check_state (guestfs_h *g, const char *caller)
 {
   if (!guestfs_is_ready (g)) {
     if (guestfs_is_config (g))
-      error (g, \"%%s: call launch() before using this function\",
+      error (g, \"%%s: call launch before using this function\\n(in guestfish, don't forget to use the 'run' command)\",
         caller);
     else if (guestfs_is_launching (g))
       error (g, \"%%s: call wait_ready() before using this function\",