Fix IPC on Forge 1.17.1 - 1.20.1 (#4187)

* Reapply "Implement a more robust IPC system between the launcher and client (#4159)"

This reverts commit e25d726da4.

* Put game JAR and theseus JAR ahead of other JARs in classpath

* Fix 1.17-1.20 Forge by forcefully removing Multi-Release manifest entry

* Fix formatting

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Co-authored-by: Prospector <6166773+Prospector@users.noreply.github.com>

packages/app-lib/src/error.rs

@@ -151,6 +151,9 @@ pub enum ErrorKind {
         "A skin texture must have a dimension of either 64x64 or 64x32 pixels"
     )]
     InvalidSkinTexture,
+
+    #[error("RPC error: {0}")]
+    RpcError(String),
 }
 
 #[derive(Debug)]