Renumerate IPROTO error codes

This patch changes iproto codes enumeration scheme:

* 0 - OK (indicates success result of DML or ADMIN commands)
* [    1, 128) - DML & ADMIN commands
* [32768, 65535) - errors (1<< 15 | ER_XXX from errcode.h)

Other ranges are reserved for future use.
Merge branch 'gh-344-iproto-error-codes-v2'

Conflicts:
	src/box/replica.cc
	src/box/txn.cc
	src/iproto_constants.cc
	src/iproto_constants.h
	test/box/bad_trigger.result
	test/box/call.result
	test/lib/tarantool-python