patch 8.2.1444: error messages are spread out and names can be confusing

Problem:    Error messages are spread out and names can be confusing.
Solution:   Start moving error messages to a separate file and use clear
            names.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 85d4df6..87ae5f9 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -1789,7 +1789,7 @@
 	    --ea.cmd;
 	else if (ea.cmd > cmd)
 	{
-	    emsg(_(e_colon_required));
+	    emsg(_(e_colon_required_before_a_range));
 	    goto doend;
 	}
 	p = find_ex_command(&ea, NULL, lookup_scriptvar, NULL);