disallow opening private files (#7165)

- Disallow sharing gitignored files through collab
- Show errors when failing to open files
- Show a warning to followers when view is unshared

/cc @mikaylamaki, let's update this to use your `private_files` config
before merge.


Release Notes:

- Added the ability to prevent sharing private files over collab.

---------

Co-authored-by: Piotr <piotr@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>

crates/rpc/proto/zed.proto

@@ -216,6 +216,7 @@ enum ErrorCode {
     BadPublicNesting = 9;
     CircularNesting = 10;
     WrongMoveTarget = 11;
+    UnsharedItem = 12;
 }
 
 message Test {

crates/rpc/src/error.rs

@@ -80,6 +80,8 @@ pub trait ErrorExt {
     fn error_tag(&self, k: &str) -> Option<&str>;
     /// to_proto() converts the error into a proto::Error
     fn to_proto(&self) -> proto::Error;
+    ///
+    fn cloned(&self) -> anyhow::Error;
 }
 
 impl ErrorExt for anyhow::Error {
@@ -106,6 +108,14 @@ impl ErrorExt for anyhow::Error {
             ErrorCode::Internal.message(format!("{}", self)).to_proto()
         }
     }
+
+    fn cloned(&self) -> anyhow::Error {
+        if let Some(rpc_error) = self.downcast_ref::<RpcError>() {
+            rpc_error.cloned()
+        } else {
+            anyhow::anyhow!("{}", self)
+        }
+    }
 }
 
 impl From<proto::ErrorCode> for anyhow::Error {
@@ -189,6 +199,10 @@ impl ErrorExt for RpcError {
             tags: self.tags.clone(),
         }
     }
+
+    fn cloned(&self) -> anyhow::Error {
+        self.clone().into()
+    }
 }
 
 impl std::error::Error for RpcError {