Parsing Error Defaulting to CHOOSE

[sepehr541]

When parsing the following, the parser correctly throws an error, but the error message notes that a CHOOSE expression is expected.
Not sure if there is a right default, but the essence of the issue here is the extra \, not a wrong keyword/expression.

---- MODULE Test ----
VARIABLES a
      Op ==
        /\\ a' = 1
====

 pgo.parser.ParseFailureError: 
  parsing failed: 'CHOOSE' expected but '\' found at <specFile>:5.11
          /\\ a' = 1
            ^
    pgo.parser.ParseFailureError$.apply(ParsingErrors.scala:117)
    pgo.parser.ParsingUtils.checkResult(ParsingUtils.scala:24)
    pgo.parser.ParsingUtils.checkResult$(ParsingUtils.scala:8)
    pgo.parser.TLAParser$.checkResult(TLAParser.scala:1362)
    pgo.parser.TLAParser$.readModule(TLAParser.scala:1389)
    compiler.Main$.parseTLAModule(Compiler.scala:154)
    compiler.FooTests$.checkEqual(CompilerTests.scala:17)
    compiler.FooTests$.tests$$anonfun$1$$anonfun$18(CompilerTests.scala:346)

[fhackett]

This is to do with the "most recent error" mechanic for how Scala Parser Combinators "remembers" errors. CHOOSE expressions are the last in the list to try, so they get flagged.

I suppose it would make sense to relabel that error as "expected valid expression" or something.