patch 8.2.4978: no error if engine selection atom is not at the start

Problem:    No error if engine selection atom is not at the start.
Solution:   Give an error. (Christian Brabandt, closes #10439)
diff --git a/src/errors.h b/src/errors.h
index 526c5f7..7a21cee 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3277,3 +3277,5 @@
 EXTERN char e_illegal_character_in_word[]
 	INIT(= N_("E1280: Illegal character in word"));
 #endif
+EXTERN char e_atom_engine_must_be_at_start_of_pattern[]
+	INIT(= N_("E1281: Atom '\\%%#=%c' must be at the start of the pattern"));