tor-error: Add ErrorKinds for RPC errors.

These are experimental for now so that we can change them without
breaking semver.

crates/tor-error/Cargo.toml

@@ -13,7 +13,8 @@ categories = ["rust-patterns"]
 
 [features]
 default = ["backtrace"]
-experimental-api = []
+experimental-api = ["rpc"]
+rpc = []
 
 [dependencies]
 backtrace = { version = "0.3.39", optional = true }

crates/tor-error/src/lib.rs

@@ -614,6 +614,29 @@ pub enum ErrorKind {
     #[display(fmt = "possible clock skew detected")]
     ClockSkew,
 
+    /// An RPC operation failed because a request could not be parsed or was
+    /// otherwise invalid.
+    #[display(fmt = "invalid RPC request")]
+    #[cfg(feature = "rpc")]
+    RpcInvalidRequest,
+
+    /// An RPC operation failed because a method type could not be found,
+    /// or is not available on a given object.
+    #[display(fmt = "RPC method not found")]
+    #[cfg(feature = "rpc")]
+    RpcMethodNotFound,
+
+    /// An RPC operation failed because the method type's parameters were not
+    /// correct for it.
+    #[display(fmt = "RPC invalid parameters")]
+    #[cfg(feature = "rpc")]
+    RpcInvalidMethodParameters,
+
+    /// An RPC operation failed because a given object could not be found.
+    #[display(fmt = "RPC object not found")]
+    #[cfg(feature = "rpc")]
+    RpcObjectNotFound,
+
     /// Internal error (bug) in Arti.
     ///
     /// A supposedly impossible problem has arisen.  This indicates a bug in