Better error message for missing optional-groups access in Regex

[spalladino]

The following code fails with a Nil assertion failed:

/(A)?B/ =~ "B"
p $1

Full stack trace being:

Nil assertion failed (Exception)
0x10f4205a2: *CallStack::unwind:Array(Pointer(Void)) at ??
0x10f420541: *CallStack#initialize:Array(Pointer(Void)) at ??
0x10f420518: *CallStack::new:CallStack at ??
0x10f41c521: *raise<Exception>:NoReturn at ??
0x10f41c501: *raise<String>:NoReturn at ??
0x10f4331f3: *Nil#not_nil!:NoReturn at ??
0x10f44d34e: *Regex::MatchData#[]<Int32>:String at ??
0x10f41c0d0: __crystal_main at ??
0x10f41f958: main at ??

This happens because the first capturing group was optional, and not present in the current match, so when tried to be cast to String it fails. Note that using String? works.

However, the error message should be more descriptive (similar to the Missing hash key when accessing a hash) to avoid confusion. See #3971 for example.

[akzhan]

I believe that this issue should be closed.

[asterite]

Indeed, this seems to be fixed. @akzhan Thank you!