Ensure we import `errno` in ts_library.py
Ensure we import `errno` in ts_library.py

Without this change I was seeing an error when compiling TS:

```
  File "../../third_party/typescript/ts_library.py", line 119, in copy_all_typescript_sources
    if exc.errno == errno.EEXIST and os.path.isdir(path):
NameError: global name 'errno' is not defined
```

Change-Id: I16654d1a3d0d89346957599150c877b44f734ccc
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2078925
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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