Should have a better error message when [unit]type= in ActionWML names an unknown type

[stevecotton]

Describe the desired feature

In 1.18.0+dev 545ba26, trying to create a [unit] with an unknown type from ActionWML gives an uninformative error message, to both stdout and the in-game chat:

<Lua caught unknown exception>

In 1.16 it still gave a Lua error, but named the unit type (to both stdout and the in-game chat):

error scripting/lua: game_error: unknown unit type: AE_mag_Mystical_Toadxxxxxx
stack traceback:
	[C]: in local 'cmd'
	lua/wml-utils.lua:144: in field 'handle_event_commands'
	lua/wml-flow.lua:5: in function <lua/wml-flow.lua:4>

The information is available to 1.18, because the exception is thrown from src/units/unit.cpp:223:

if(!i) throw unit_type::error("unknown unit type: " + type_id);

but it ends up here at src/scripting/lua_common.cpp:1171:

ERR_LUA << "Lua caught unknown exception";

[soliton-]

Sounds like it might be related to #8234.