lark-js should advise user when `--start` not passed rather than outputting a cryptic error

[turon]

Trying to output a js parser for this matter.lark file fails with the following error:

$ ~/.local/bin/lark-js matter_grammar.lark -o matter_grammar.js
...
lark.exceptions.GrammarError: Using an undefined rule: NonTerminal('start')

Yet solving this is as simple as passing the correct start symbol with the -s flag with idl start symbol used by this .lark file:

$ ~/.local/bin/lark-js matter_grammar.lark -s idl -o matter_grammar.js

The lark-js should advise the user that the .lark file has no start symbol, and that the correct start symbol should be passed via --start rather than dumping a raw error backtrace.