Fix "usage" errors for some MI varobj commands

I noticed that the "usage" error for -var-set-frozen mentioned the
wrong command name.  Then I looked through the whole file and found a
couple other spots that didn't mention the command name at all.  This
patch fixes all of these.

Reviewed-by: Kevin Buettner <[email protected]>

Author: tromey

gdb/mi/mi-cmd-var.c

@@ -246,7 +246,7 @@ mi_cmd_var_set_visualizer (const char *command, const char *const *argv,
   struct varobj *var;
 
   if (argc != 2)
-    error (_("Usage: NAME VISUALIZER_FUNCTION."));
+    error (_("-var-set-visualizer: Usage: NAME VISUALIZER_FUNCTION."));
 
   var = varobj_get_handle (argv[0]);
 
@@ -263,7 +263,7 @@ mi_cmd_var_set_frozen (const char *command, const char *const *argv, int argc)
   bool frozen;
 
   if (argc != 2)
-    error (_("-var-set-format: Usage: NAME FROZEN_FLAG."));
+    error (_("-var-set-frozen: Usage: NAME FROZEN_FLAG."));
 
   var = varobj_get_handle (argv[0]);
 
@@ -442,7 +442,7 @@ mi_cmd_var_info_path_expression (const char *command, const char *const *argv,
   struct varobj *var;
 
   if (argc != 1)
-    error (_("Usage: NAME."));
+    error (_("-var-info-path-expression: Usage: NAME."));
 
   /* Get varobj handle, if a valid var obj name was specified.  */
   var = varobj_get_handle (argv[0]);