Better error messages for \x and \u

[RealyUniqueName]

Fixes #8191

X1.hx:2: character 11 : Invalid escape sequence \x. Must be followed by a hexadecimal sequence.
U5.hx:2: character 11 : Invalid escape sequence \u. Maximum allowed value for unicode escape sequence is \u10FFFF

[Gama11]

Hm, very minor complaint, but Must be followed by a hexadecimal sequence doesn't seem quite correct for \u, since you also need the {}..

[RealyUniqueName]

Actually \uFFFF also works.
Can somebody propose better wording?

[Gama11]

Oh, I didn't realize that. What's the point of the {} then?

[RealyUniqueName]

According to the ml code, the non-curly version is only allowed for two-byte sequences.

[Gama11]

I see. But yeah, the message is probably fine then, definitely a big improvement to before. :)

[RealyUniqueName]

Changed to

U3.hx:2: character 11 : Invalid escape sequence \u. Must be followed by a hexadecimal sequence enclosed in curly brackets.

because of curly brackets always work.