MEDIUM: proto_htx: Convert all HTTP error messages into HTX

During startup, after the configuration parsing, all HTTP error messages
(errorloc, errorfile or default messages) are converted into HTX messages and
stored in dedicated buffers. We use it to return errors in the HTX analyzers
instead of using ugly OOB blocks.
diff --git a/include/common/http.h b/include/common/http.h
index 0835d07..66acfd3 100644
--- a/include/common/http.h
+++ b/include/common/http.h
@@ -119,6 +119,7 @@
 };
 
 extern const int http_err_codes[HTTP_ERR_SIZE];
+extern const char *http_err_msgs[HTTP_ERR_SIZE];
 extern struct buffer http_err_chunks[HTTP_ERR_SIZE];
 const struct ist http_known_methods[HTTP_METH_OTHER];
 extern const uint8_t http_char_classes[256];