patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index c5758cc..e3e1721 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4991,7 +4991,7 @@
 	}
 	if (type_is && (p[len] == '?' || p[len] == '#'))
 	{
-	    semsg(_(e_invexpr2), *arg);
+	    semsg(_(e_invalid_expression_str), *arg);
 	    return FAIL;
 	}
 	// extra question mark appended: ignore case
@@ -5821,7 +5821,7 @@
 	if (p == NULL)
 	{
 	    // cannot happen?
-	    emsg(_(e_letunexp));
+	    emsg(_(e_unexpected_characters_in_assignment));
 	    return FAIL;
 	}
 	cc = *p;