feat(gateway): add RawError option

CHANGELOG.md

@@ -17,6 +17,9 @@ The following emojis are used to highlight certain changes:
 ### Added
 
 * ✨ The `routing/http` implements Delegated Peer Routing introduced in [IPIP-417](https://github.com/ipfs/specs/pull/417).
+* An option `RawError` has been added to `gateway.Config`. When this option is `true`,
+  pretty HTML error pages are disabled. Instead, a `text/plain` page with the raw error
+  message as the body is returned.
 
 ### Changed
 

gateway/errors.go

@@ -159,7 +159,7 @@ func webError(w http.ResponseWriter, r *http.Request, c *Config, err error, defa
 		code = gwErr.StatusCode
 	}
 
-	acceptsHTML := strings.Contains(r.Header.Get("Accept"), "text/html")
+	acceptsHTML := !c.RawError && strings.Contains(r.Header.Get("Accept"), "text/html")
 	if acceptsHTML {
 		w.Header().Set("Content-Type", "text/html")
 		w.WriteHeader(code)

gateway/gateway.go

@@ -40,6 +40,11 @@ type Config struct {
 	// overridden per FQDN in PublicGateways. To be used with WithHostname.
 	NoDNSLink bool
 
+	// RawError disables pretty HTML pages when an error occurs. Instead, a `text/plain`
+	// page will be sent with the raw error message. This can be useful if this gateway
+	// is being proxied by other service, which wants to use the error message.
+	RawError bool
+
 	// PublicGateways configures the behavior of known public gateways. Each key is
 	// a fully qualified domain name (FQDN). To be used with WithHostname.
 	PublicGateways map[string]*PublicGateway