Error recovery in parser

[achudnov]

Some parse errors can be recovered from and parsing can continue. For example. expressions on the left-hand-side of an assignment can only be LeftHandSideExpressions (per the spec, see also issue #40), but it might possible to parse any expression ---even an incorrect one in the context--- and keep parsing. This could potentially allow reporting more than one syntactic error at a time, which might be useful for user-facing utilities like compilers and static analyzers.

Note that this would require substantial changes to the parser and is probably related to issue #42 .