parsed actions: Better error message on failed parse

[Pokey Rule]

The error message when the user gives a malformed custom command is not very helpful. We should probably use the VSCode output channel to show them the error that comes from our parser:

Error: invalid syntax at line 1 col 22:

1  chuck block <target> and <target>
                        ^
Unexpected input (lexer error). I did not expect any more input. Here is the state of my parse table:

    mark → %placeholderMark ● 
    primitiveTarget → primitiveTarget$ebnf$2 mark ● 
    target → primitiveTarget ● 
    action → %simpleActionName target ● 
    main → action ● 

This seems like a good place to start