This reverts commit 362ef5f6.

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 flagmask field from set_mapgen_params table
Add small bits of needed documentation

Lua does not guarantee that the indexes of a table will be in numerical order.