Fix error propagation for supervisor (#5503)

This change improves the error message for some fairly generic-looking
errors like "HTTP request failed" that are occasionally reported when
kernels fail to start.

The change may look like a no-op, but it isn't; because of the way JS
async exception handling works, we need to explicitly `await` the async
expression in order to catch exceptions. What was happening here was
that exceptions thrown from `tryStart()` in this codepath were not
getting handled in the `try/catch` block (despite being thrown from
inside the block). Instead, they were thrown directly by `start()`, with
the result that none of the code intended to marshal API errors to the
UI layer was running to summarize the error.

Partial backport of #5490 for
the 2024.11.1 branch.

extensions/positron-supervisor/src/KallichoreSession.ts

@@ -711,7 +711,8 @@ export class KallichoreSession implements JupyterLanguageRuntimeSession {
 	async start(): Promise<positron.LanguageRuntimeInfo> {
 		try {
 			// Attempt to start the session
-			return this.tryStart();
+			const info = await this.tryStart();
+			return info;
 		} catch (err) {
 			if (err instanceof HttpError && err.statusCode === 500) {
 				// When the server returns a 500 error, it means the startup