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/filepath.c b/src/filepath.c
index 69bc8b5..0e29977 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -856,7 +856,7 @@
 	// delete a directory recursively
 	rettv->vval.v_number = delete_recursive(name);
     else
-	semsg(_(e_invexpr2), flags);
+	semsg(_(e_invalid_expression_str), flags);
 }
 
 /*
@@ -1685,7 +1685,7 @@
 
     if (mch_isdir(fname))
     {
-	semsg(_(e_isadir2), fname);
+	semsg(_(e_src_is_directory), fname);
 	return;
     }
     if (*fname == NUL || (fd = mch_fopen((char *)fname, READBIN)) == NULL)