Handle failures in noticeReceiver/noticeProcessor

[daurnimator]

The current noticeReceiver and noticeProcessor use lua functions that may fail and longjmp out.

One option is to lua_pcall out to a second function.

• This could make it easier to add coroutine support (via lua_pcallk)

Alternatively, only use functions that can't fail.

• use a unique pointer and lua_rawgetp instead of lua_pushstring+lua_rawget
  • This is new in lua 5.2+
• preallocate the PGresult *
• never call luaL_error
  • A PQnoticeReceiver is by definition called for warnings or below, so it's not supposed to fail