Made many rpc:call/4,5 calls safer if rex is down.

Created a couple of utility functions to handle do the wrapping. Most
rpc:call/4,5 that where not wrapped in a try/catch have been changed to
use the utility. Those that weren't appeared to want a crash due to
context (such as explictly matching for only the success value).
Updated dialyzer ignored warnings.
Merge pull request #586 from basho/bugfix/mw/safer_rpc

Made many rpc:call/4,5 calls safer if rex is down.