app/testpmd: log mbuf pool creation

app/testpmd: log mbuf pool creation

Enhance the logs related to mbuf pool creation. Display an info level
log when creating the mbuf, and display the error as a string on failure.

After the patch, we have:

  [...]
  EAL:   probe driver: 8086:10fb rte_ixgbe_pmd
  USER1: create a new mbuf pool <mbuf_pool_socket_0>: n=331456, \
      size=2176, socket=0
  EAL: Error - exiting with code: 1
    Cause: Creation of mbuf pool for socket 0 failed: Cannot allocate \
      memory

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>