Make serializeStructToString use an ostringstream
Revert "Make sure we get a stacktrace for as many lua errors as possible"

This reverts commit 362ef5f6ced862daa4733034810d0b07e2ad5d89.

Stack tracebacks couldn't be generated in LuaError::LuaError anyway and this
caused a second, empty traceback in most cases.  In cases where there wasn't
annother traceback the stack had already unwound and the traceback was empty.
Remove lua_State parameter from LuaError::LuaError
Add more informative error messages for inventory and item method errors