make server-side RPC handler more robust
make server-side RPC handler more robust

* Check if callback actually is a function. This is not the case in
  certain situations, e.g. when using the multitransport-jsonrpc lib
  with incorrect parameters on the client-side, and has serious
  consequences: multitransport-jsonrpc rethrows the resulting Error in
  process.nextTick, which puts it outside the RC Fiber, crashing the
  GS worker process.

reversefold/eleven-server,Whizy/eleven-server,Whizy/eleven-server,KaiyonAlatar/eleven-server,yelworc/eleven-server,Whizy/eleven-server,reversefold/eleven-server,ElevenGiants/eleven-server,yelworc/eleven-server,KaiyonAlatar/eleven-server,ElevenGiants/eleven-server,Mexiswow/eleven-server,ElevenGiants/eleven-server,KaiyonAlatar/eleven-server,Mexiswow/eleven-server