Improve Langium parser errors

[ym-han]

• https://github.com/eclipse-langium/langium/pull/1108
• https://github.com/eclipse-langium/langium/blob/550798780c83b8cf24a411a5d0a295263948e1d9/packages/langium/src/parser/langium-parser.ts#L438
• https://chevrotain.io/docs/features/custom_errors.html
• https://github.com/chevrotain/chevrotain/blob/master/examples/parser/custom_errors/custom_errors.js

Might also be worth asking Inari what kinds of error messages she thinks would be helpful.

Also, helpful note from https://github.com/eclipse-langium/langium/discussions/1045 -- it's better for the grammar to be more permissive; validators can be used to rein in undesired generations and provide better error messages