Parse() can throw unexpected exceptions about xfunc

[sys27]

Currently TokenizeExcetion/ParseException is thrown only when Lexer/Parse detects incorrect input. For example, the last case doesn't result in StackOverflow but instead throws ParseException because it detects invalid grammar.

First two cases are different, they are based on methods from the standard .NET library. They may throw FormatException or OverflowException.
FormatException - it tries to parse 2e as "scientific notation", like 2e+10 but the last part is missing, so here is an exception. An additional point to think about: xFunc supports implicit multiplication, like 2x. So, in that case, 2e could be parsed as 2 * e, maybe we need to validate it.
OverflowException - completely valid case. xFunc supports different numeral systems (dec, oct - number start from 0, hex - 0x prefix, bin - 0b prefix), and oct/hex/bin are only supported for integer numbers. So, your number is treated as octal and it is too big to be stored in long.

Unfortunately, these exceptions aren't handled by xFunc. I agree it is better to wrap them into a custom exception. #767