Print the invocation site for @errors in mixins and functions

[nex3]

Currently, if an @error is encountered in a mixin or a function, the source span that's associated with that error (and thus the snippet that's printed in error messages) is the @error rule itself. This isn't super useful; @error is usually used to validate calls to an API, so if it fails it should show the call site instead.

The stack trace shouldn't be affected.

For example:

@mixin validate-arg($arg) {
  @if $arg < 10 {
    @error "Arg #{$arg} should be >= 10.";
  }
}

.foo {
  @include validate-arg(9);
}

Currently prints:

Error: "Arg 9 should be >= 10."
    @error "Arg #{$arg} should be >= 10.";
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  test.scss 3:5  validate-arg()
  test.scss 8:3  root stylesheet

but it should print:

Error: "Arg 9 should be >= 10."
    @include validate-arg(9);
    ^^^^^^^^^^^^^^^^^^^^^^^^
  test.scss 3:5  validate-arg()
  test.scss 8:3  root stylesheet

[nex3]

@jmesserly do you want to give this a shot?

[jmesserly]

thanks, yes!