eal: do not panic on interrupt thread init

eal: do not panic on interrupt thread init

There could be some confusion as to why the call failed - this change
will always reflect the value of the error in rte_error.

When initializing the interrupt thread, there are a number of possible
reasons for failure - some of which are correctable by the application.
Do not panic() needlessly, and give the application a change to reflect
this information to the user.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>