Why do failed contracts don't dump stack backtrace in unittests?

[Ali Çehreli via Digitalmars-d-learn]

The program output is different whether an Error is thrown from main or 
from the unittest block:

void foo(string s)
in (s != "hello") {
}
unittest {
  foo("hello");  // No stack backtrace
}

void main() {
  foo("hello");  // Yes stack backtrace
}

Ali