error functions: return an int

Stop returning a void for functions, future-proofing them to allow them
to fail.

include/git2/errors.h

@@ -143,8 +143,9 @@ GIT_EXTERN(void) git_error_clear(void);
  * @param error_class One of the `git_error_t` enum above describing the
  *                    general subsystem that is responsible for the error.
  * @param string The formatted error message to keep
+ * @return 0 on success or -1 on failure
  */
-GIT_EXTERN(void) git_error_set_str(int error_class, const char *string);
+GIT_EXTERN(int) git_error_set_str(int error_class, const char *string);
 
 /**
  * Set the error message to a special value for memory allocation failure.

src/errors.c

@@ -95,19 +95,25 @@ void git_error_vset(int error_class, const char *fmt, va_list ap)
 		set_error_from_buffer(error_class);
 }
 
-void git_error_set_str(int error_class, const char *string)
+int git_error_set_str(int error_class, const char *string)
 {
 	git_buf *buf = &GIT_GLOBAL->error_buf;
 
 	assert(string);
 
-	if (!string)
-		return;
+	if (!string) {
+		git_error_set(GIT_ERROR_INVALID, "unspecified caller error");
+		return -1;
+	}
 
 	git_buf_clear(buf);
 	git_buf_puts(buf, string);
-	if (!git_buf_oom(buf))
-		set_error_from_buffer(error_class);
+
+	if (git_buf_oom(buf))
+		return -1;
+
+	set_error_from_buffer(error_class);
+	return 0;
 }
 
 void git_error_clear(void)