Logging: capture and re-throw more clear error when sfl4j not in classpath but logging enabled.

[zhumin8]

context: #3403 (comment)

For scenario where customer app does not have logging dependency (SLF4J), but turns on logging via env variable, logging would fail. Currently, this would throw ClassNotFoundException.

For better customer experience, we can capture this and re-throw with more clear error message to communicate to customer.

Note: if change, this applies to both Gax and Auth. Need code changes in both repos.