reftable: add error related functionality

The reftable/ directory is structured as a library, so it cannot
crash on misuse. Instead, it returns an error code.

In addition to signaling errors, the error code can be used to signal
conditions from lower levels of the library to be handled by higher
levels of the library. For example, in a transaction we might
legitimately write an empty reftable file, but in that case, we want to
shortcut the transaction.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/error.c

@@ -0,0 +1,41 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "reftable-error.h"
+
+#include <stdio.h>
+
+const char *reftable_error_str(int err)
+{
+	static char buf[250];
+	switch (err) {
+	case REFTABLE_IO_ERROR:
+		return "I/O error";
+	case REFTABLE_FORMAT_ERROR:
+		return "corrupt reftable file";
+	case REFTABLE_NOT_EXIST_ERROR:
+		return "file does not exist";
+	case REFTABLE_LOCK_ERROR:
+		return "data is outdated";
+	case REFTABLE_API_ERROR:
+		return "misuse of the reftable API";
+	case REFTABLE_ZLIB_ERROR:
+		return "zlib failure";
+	case REFTABLE_NAME_CONFLICT:
+		return "file/directory conflict";
+	case REFTABLE_EMPTY_TABLE_ERROR:
+		return "wrote empty table";
+	case REFTABLE_REFNAME_ERROR:
+		return "invalid refname";
+	case -1:
+		return "general error";
+	default:
+		snprintf(buf, sizeof(buf), "unknown error code %d", err);
+		return buf;
+	}
+}