Fixes#23643.
We weren't catching the cancellation exception thrown by TSC on the JS
side, so the rust side was catching this exception and then attempting
to print out the exception via `toString`. That last bit resulted in a
cryptic `[object Object]` showing up in the logs like so:
```
Error during TS request "getCompletionEntryDetails":
[object Object]
```
I'm not 100% sure how we weren't seeing this in the past. My guess is
that #23409 and the subsequent PR to improve the exception catching and
logging surfaced this, but I'm still not quite clear on it.
My initial fix here returned `null` to rust when a server request was
cancelled, but this resulted in a deserialization error when we
attempted to deserialize that into the expected response type. So now,
as soon as the request's cancellation token signals we'll stop waiting
for a response and return an error (which will get swallowed as the LSP
request is being cancelled).
I was a bit surprised to find that [this
branch](0c671c9792/cli/lsp/tsc.rs (L1093))
actually executes sometimes, I believe due to the fact that aborting a
future may not [immediately stop its
execution](https://docs.rs/futures/latest/futures/stream/struct.AbortHandle.html#method.abort).
66b66de96a
