Uncaught Exception/Rejection from Gax

[alkatrivedi]

While working on TPC support for the nodejs client we came across this bug.

Error thrown from Gax libraries are not handled on the client side which is resulting into unhandled exception/rejection.

Steps to reproduce:

1. Refer to the autogenerated Nodejs client code: https://github.com/googleapis/nodejs-spanner/blob/main/src/v1/spanner_client.ts#L119-#L127
2. Set both the universe_domain and universeDomain with different values within SpannerOption.

An error is subsequently thrown from the specified lines of code but is not being caught on the client side. Due to the asynchronous implementation, this uncaught error is causing the code execution to half indefinitely.

[alkatrivedi]

cc: @surbhigarg92