common/OpTestTConnection: use exception rather than assert
common/OpTestTConnection: use exception rather than assert

asserts can be compiled out of bytecode, so instead let's use
an exception to at least give some sort of feedback to the user
in this case this case.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework