fetchActions: Pull await backoffMachine.wait() out of catch block.

As Greg points out [1], this makes the most sense conceptually; it's
happening at the bottom of the loop, just before a new iteration
starts. The `return` in the `try` block is enough to ensure that
this wait won't interfere with a successful fetch.

[1]: zulip#4166 (comment)

src/message/fetchActions.js

@@ -258,8 +258,8 @@ export async function tryFetch<T>(func: () => Promise<T>): Promise<T> {
           if (isClientError(e)) {
             throw e;
           }
-          await backoffMachine.wait();
         }
+        await backoffMachine.wait();
       }
       // Without this, Flow 0.92.1 does not know this code is unreachable,
       // and it incorrectly thinks Promise<undefined> could be returned,