patch 8.2.3961: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.
diff --git a/src/filepath.c b/src/filepath.c
index f4bddf8..a9edf7d 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -842,7 +842,7 @@
     name = tv_get_string(&argvars[0]);
     if (name == NULL || *name == NUL)
     {
-	emsg(_(e_invarg));
+	emsg(_(e_invalid_argument));
 	return;
     }
 
@@ -2267,7 +2267,7 @@
     }
     else
     {
-	semsg(_(e_invarg2),
+	semsg(_(e_invalid_argument_str),
 		_("writefile() first argument must be a List or a Blob"));
 	return;
     }