bridge config: Feature-gate the error variants

This leaves this enum empty of actual errors, when bridge-client is
disabled.

We're going to add the not supported variant in a moment.

crates/tor-guardmgr/src/bridge/config/err.rs

@@ -10,10 +10,12 @@ use thiserror::Error;
 #[non_exhaustive]
 pub enum BridgeParseError {
     /// Bridge line was empty
+    #[cfg(feature = "bridge-client")]
     #[error("Bridge line was empty")]
     Empty,
 
     /// Expected PT name or host:port, looked a bit like a PT name, but didn't parse
+    #[cfg(feature = "bridge-client")]
     #[error(
         "Cannot parse {word:?} as PT name ({pt_error}), nor as direct bridge IpAddress:ORPort"
     )]
@@ -25,6 +27,7 @@ pub enum BridgeParseError {
     },
 
     /// Expected PT name or host:port, looked a bit like a host:port, but didn't parse
+    #[cfg(feature = "bridge-client")]
     #[error(
         "Cannot parse {word:?} as direct bridge IpAddress:ORPort ({addr_error}), nor as PT name"
     )]
@@ -47,6 +50,7 @@ pub enum BridgeParseError {
     },
 
     /// Cannot parse value as identity key, or PT key=value
+    #[cfg(feature = "bridge-client")]
     #[error("Cannot parse {word:?} as identity key ({id_error}), or PT key=value")]
     InvalidIdentityOrParameter {
         /// The offending word
@@ -75,6 +79,7 @@ pub enum BridgeParseError {
     },
 
     /// More than one identity of the same type specified
+    #[cfg(feature = "bridge-client")]
     #[error("More than one identity of the same type specified, at {word:?}")]
     MultipleIdentitiesOfSameType {
         /// The offending word
@@ -82,6 +87,7 @@ pub enum BridgeParseError {
     },
 
     /// Identity specified of unsupported type
+    #[cfg(feature = "bridge-client")]
     #[error("Identity specified but not of supported type, at {word:?}")]
     UnsupportedIdentityType {
         /// The offending word
@@ -89,15 +95,18 @@ pub enum BridgeParseError {
     },
 
     /// Parameters may only be specified with a pluggable transport
+    #[cfg(feature = "bridge-client")]
     #[error("Parameters supplied but not valid without a pluggable transport")]
     DirectParametersNotAllowed,
 
     /// Every bridge must have an RSA identity
+    #[cfg(feature = "bridge-client")]
     #[error("Bridge line lacks specification of RSA identity key")]
     NoRsaIdentity,
 
     /// Pluggable transport support disabled in cargo features
     // We deliberately make this one *not* configured out if PT support is enabled
+    #[cfg(feature = "bridge-client")]
     #[error("Pluggable transport requested ({word:?} is not an IpAddress:ORPort), but support disabled in cargo features")]
     PluggableTransportsNotSupported {
         /// The offending word