[runtime/observatory] Clean up rewind command feedback

This check is also replicated later within [debugger.cc](https://github.com/dart-lang/sdk/blob/master/runtime/vm/debugger.cc#L4098), so it didn't necessarily matter that this check did not work.

Despite this, I think the earlier check does not hurt though, and I
also added feedback for when the supplied argument is not an int
rather than just throwing an error.

Bug: https://github.com/dart-lang/sdk/issues/38888
Change-Id: I8a7a237850957d13dbf0aa30873d3b9772eb6d1a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148567
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

https://dart.googlesource.com/sdk/+/1fba93f7c72eefae3701a63381aab521ffd79729
1 file changed
tree: 608b9b8c9629e8e54c76bb31892e86b078dbe210
  1. commits.json