[PH] Trx Generator - Use default exception handler.

[oschwaldp-oci]

The non-default exception handler was masking the issue and not terminating the process which was causing misleading errors downstream. With default exception handler there is a better exception message as well as termination of the process. This provides better handling upstream in the performance harness.

This was being caused by an http response of ok with an empty string response body.

Using the default exception handler, the exception now looks like:

Fail to parse JSON from string:
    {"string":""}
    provider_connection-0  trx_provider.cpp:172 operator()
unexpected end of file
    {}
    provider_connection-0  json.cpp:435 variant_from_stream

    {"str":""}
    provider_connection-0  json.cpp:461 from_string

terminate called after throwing an instance of 'eosio::chain::json_parse_exception'
  what():  JSON parse exception

Which terminates the trx_generator process and proper exit return code is passed back to the PerformanceHarness.

Relates to ongoing work in #1662