Improve handling of poolboy timeouts during ping requests

The design of the riak_cs_wm_ping webmachine resource is that it
blocks for a timeout period while attempting to checkout a Riak
connection from the poolboy request_pool. If the timeout expires
before a pool connection is checked out it is designed to attempt to
establish a direct connection to Riak and then attempt the ping.

The reality of what occurs is that the process handling the ping request
crashes after the timeout expires. This is due to the fact that
poolboy:checkout calls gen_fsm:sync_send_event with a timeout
parameter specified. The timeout expiration results in a call to
exit(timeout) that causes the request process to crash and return a
500 error to the client.

This change modifies the riak_cs_wm_ping resource to catch the exit on
timeout and return the atom full. This allows the direct connection
logic to execute and either return success to the user or a 503 error
indicating the system is too heavily loaded.
Merge branch 'bugfix/ping-timeout-handling' into develop