Enhance error handling in gateway and configuration

- Added `expose_upstream_errors` option to the configuration, allowing detailed error messages in JSON responses for 502 errors.
- Implemented `writeBadGatewayJSON` function to streamline JSON error responses, including upstream error details when enabled.
- Updated documentation to reflect changes in configuration and error handling behavior for improved diagnostics.

internal/config/config.go

@@ -15,8 +15,10 @@ var aliasRe = regexp.MustCompile(`^[a-z0-9][a-z0-9_-]*$`)
 
 // Config is the gateway YAML configuration.
 type Config struct {
-	Listen             string           `yaml:"listen"`
-	AllowAll           bool             `yaml:"allow_all"`
+	Listen string `yaml:"listen"`
+	// ExposeUpstreamErrors puts RoundTrip error text in JSON 502 bodies (error.detail). For private ops only.
+	ExposeUpstreamErrors bool `yaml:"expose_upstream_errors"`
+	AllowAll             bool `yaml:"allow_all"`
 	WhitelistCIDRs     []string         `yaml:"whitelist_cidrs"`
 	TrustedProxies     []string         `yaml:"trusted_proxies"`
 	CorsAllowedOrigins []string         `yaml:"cors_allowed_origins"`