Raise human-readable error in case of incorrect usage of `_on` or `_` special variables in expressions

[Mingun]

Fix the internal compiler errors in case of incorrect usages, as in this example:

meta:
  id: internal_errors
seq:
  - id: value1
    type: u1
    if: _
  - id: value2
    type: u1
    if: _on

Currently stable WebIDE shows this not much useful error:

Parse error: undefined
Call stack: undefined java.util.NoSuchElementException: None.get

[Mingun]

@GreyCat, @generalmimon, I see that you have some activity in the project recently. Could you find a time to review my PRs, for example, this?