Move error reporting into its own source file

This doesn't really seem to belong in save_git.c.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/CMakeLists.txt

@@ -40,6 +40,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	divesite.cpp
 	divelist.c
 	equipment.c
+	errorhelper.c
 	file.c
 	gas-model.c
 	git-access.c

core/errorhelper.c

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifdef __clang__
+// Clang has a bug on zero-initialization of C structs.
+#pragma clang diagnostic ignored "-Wmissing-field-initializers"
+#endif
+#include "dive.h"
+#include "membuffer.h"
+
+#define VA_BUF(b, fmt) do { va_list args; va_start(args, fmt); put_vformat(b, fmt, args); va_end(args); } while (0)
+
+static struct membuffer error_string_buffer = { 0 };
+/*
+ * Note that the act of "getting" the error string
+ * buffer doesn't de-allocate the buffer, but it does
+ * set the buffer length to zero, so that any future
+ * error reports will overwrite the string rather than
+ * append to it.
+ */
+const char *get_error_string(void)
+{
+	const char *str;
+
+	if (!error_string_buffer.len)
+		return "";
+	str = mb_cstring(&error_string_buffer);
+	error_string_buffer.len = 0;
+	return str;
+}
+
+int report_error(const char *fmt, ...)
+{
+	struct membuffer *buf = &error_string_buffer;
+
+	/* Previous unprinted errors? Add a newline in between */
+	if (buf->len)
+		put_bytes(buf, "\n", 1);
+	VA_BUF(buf, fmt);
+	mb_cstring(buf);
+
+	return -1;
+}
+
+void report_message(const char *msg)
+{
+	(void)report_error("%s", msg);
+}

core/save-git.c

@@ -440,42 +440,6 @@ static void create_dive_buffer(struct dive *dive, struct membuffer *b)
 	save_dive_temperature(b, dive);
 }
 
-static struct membuffer error_string_buffer = { 0 };
-
-/*
- * Note that the act of "getting" the error string
- * buffer doesn't de-allocate the buffer, but it does
- * set the buffer length to zero, so that any future
- * error reports will overwrite the string rather than
- * append to it.
- */
-const char *get_error_string(void)
-{
-	const char *str;
-
-	if (!error_string_buffer.len)
-		return "";
-	str = mb_cstring(&error_string_buffer);
-	error_string_buffer.len = 0;
-	return str;
-}
-
-int report_error(const char *fmt, ...)
-{
-	struct membuffer *buf = &error_string_buffer;
-
-	/* Previous unprinted errors? Add a newline in between */
-	if (buf->len)
-		put_bytes(buf, "\n", 1);
-	VA_BUF(buf, fmt);
-	mb_cstring(buf);
-	return -1;
-}
-
-void report_message(const char *msg)
-{
-	(void)report_error("%s", msg);
-}
 
 /*
  * libgit2 has a "git_treebuilder" concept, but it's broken, and can not