devserver: move DevServerError to a separate module

The exception may be used by multiple applications in future, so move it
to a separate module.

BUG=chromium:993621
TEST=started devserver and verified nothing looks wrong.

Change-Id: I669be55c222f031f764da58f3b9939ff4c3a3652
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1761785
Tested-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
Commit-Queue: Congbin Guo <guocb@chromium.org>
Auto-Submit: Congbin Guo <guocb@chromium.org>
3 files changed