Factor out olm_error_to_string to a separate file

I want to be able to use this functionality from elsewhere, so factor it out to
its own file.

include/olm/error.h

@@ -31,8 +31,15 @@ enum OlmErrorCode {
     OLM_BAD_ACCOUNT_KEY = 8, /*!< The supplied account key is invalid */
     OLM_UNKNOWN_PICKLE_VERSION = 9, /*!< The pickled object is too new */
     OLM_CORRUPTED_PICKLE = 10, /*!< The pickled object couldn't be decoded */
+
+    /* remember to update the list of string constants in error.c when updating
+     * this list. */
+
 };
 
+/** get a string representation of the given error code. */
+const char * _olm_error_to_string(enum OlmErrorCode error);
+
 #ifdef __cplusplus
 } // extern "C"
 #endif

src/error.c

+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "olm/error.h"
+
+static const char * ERRORS[] = {
+    "SUCCESS",
+    "NOT_ENOUGH_RANDOM",
+    "OUTPUT_BUFFER_TOO_SMALL",
+    "BAD_MESSAGE_VERSION",
+    "BAD_MESSAGE_FORMAT",
+    "BAD_MESSAGE_MAC",
+    "BAD_MESSAGE_KEY_ID",
+    "INVALID_BASE64",
+    "BAD_ACCOUNT_KEY",
+    "UNKNOWN_PICKLE_VERSION",
+    "CORRUPTED_PICKLE",
+};
+
+const char * _olm_error_to_string(enum OlmErrorCode error)
+{
+    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
+        return ERRORS[error];
+    } else {
+        return "UNKNOWN_ERROR";
+    }
+}

src/olm.cpp

@@ -164,20 +164,6 @@ std::size_t b64_input(
     return raw_length;
 }
 
-static const char * ERRORS[11] {
-    "SUCCESS",
-    "NOT_ENOUGH_RANDOM",
-    "OUTPUT_BUFFER_TOO_SMALL",
-    "BAD_MESSAGE_VERSION",
-    "BAD_MESSAGE_FORMAT",
-    "BAD_MESSAGE_MAC",
-    "BAD_MESSAGE_KEY_ID",
-    "INVALID_BASE64",
-    "BAD_ACCOUNT_KEY",
-    "UNKNOWN_PICKLE_VERSION",
-    "CORRUPTED_PICKLE",
-};
-
 } // namespace
 
 
@@ -192,35 +178,23 @@ size_t olm_error() {
 const char * olm_account_last_error(
     OlmAccount * account
 ) {
-    unsigned error = unsigned(from_c(account)->last_error);
-    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
-        return ERRORS[error];
-    } else {
-        return "UNKNOWN_ERROR";
-    }
+    auto error = from_c(account)->last_error;
+    return _olm_error_to_string(error);
 }
 
 
 const char * olm_session_last_error(
     OlmSession * session
 ) {
-    unsigned error = unsigned(from_c(session)->last_error);
-    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
-        return ERRORS[error];
-    } else {
-        return "UNKNOWN_ERROR";
-    }
+    auto error = from_c(session)->last_error;
+    return _olm_error_to_string(error);
 }
 
 const char * olm_utility_last_error(
     OlmUtility * utility
 ) {
-    unsigned error = unsigned(from_c(utility)->last_error);
-    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
-        return ERRORS[error];
-    } else {
-        return "UNKNOWN_ERROR";
-    }
+    auto error = from_c(utility)->last_error;
+    return _olm_error_to_string(error);
 }
 
 size_t olm_account_size() {