[Issue 19172] New: Improve betterC assertion error mesage for switch_error

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19172

          Issue ID: 19172
           Summary: Improve betterC assertion error mesage for
                    switch_error
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: greensunny12 at gmail.com

See also: https://github.com/dlang/druntime/pull/2274#issuecomment-413480569

tl;dr: In -betterC it's currently hard to do custom formatting and thus a more
sophisticated error message still needs to be generated for `switch_error`.

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