\right} isn't recognized as an error

[ForNeVeR]

Extracted from #271.

It's been decided that we shouldn't consider \left{ 2 + 2 \right} as a valid formula: \left{ and \right} should've been replaced with '\left{and\right}`.

But yet, we successfully parse a formula \left\{ 2 + 2 \right}, which shouldn't be the case.