Rescue unexpected parser errors and display source

[Morriar]

Sometimes the parser crashes because of some RBI input we never though about.

This happened today and had to figure which one of the 400,000 lines in the input RBI file was the culprit.

To make easier to debug, this PR proposes to catch all unexpected errors originating from the parser and display this:

##################################
### RBI::Parser internal error ###
##################################

There was an internal parser error while processing this source.
Please open an issue at https://github.com/Shopify/rbi/issues/new.

Error: undefined method `children' for nil:NilClass while parsing `test.rbi:4:2-6:19`:
~~~
  sig {params(
  )
  .returns(Return)}
  def call()
~~~

##################################

Shopify/rbi/lib/rbi/parser.rb:663:in `visit_send': undefined method `children' for nil:NilClass (NoMethodError)
Shopify/rbi/lib/rbi/parser.rb:113:in `rescue in parse': undefined method `children' for nil:NilClass (RBI::UnexpectedParserError)