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SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

////
/// Here is a way to use `@error` while still supporting Sass 3.3
/// and below, which have no support for the `@error` directive.
////

/// Log a message either with `@error` if supported 
/// else with `@warn`, using `function-exists('at-error')`
/// to detect support.
/// 
/// @param {String} $message - Message to log
@function log($message) {
  // The `@warn` version needs to come before `@error` because
  // Sass 3.3- chokes on `@error` inside a function as it is
  // an unrecognized directive. I believe these engines should 
  // not even see the `@error` rule since they do not enter the
  // `@else` condition however it seems they do, for some reasons. 
  // Maybe Sass is performing some quick evaluation to crash early
  // for cases like this. I'm not sure.
  @if feature-exists('at-error') != true {
    @warn $message;
  } @else {
    @error $message;
  }
}

/// Log a message either with `@error` if supported or with `@warn`.
/// 
/// @param {String} $message - Message to log
@mixin log($message) {
  // Because functions cannot be called anywhere in Sass, 
  // we need to hack the call in a dummy condition.
  // There are other ways to do this, such as `dummy: log(..)`.
  @if log($message) {}
}

// Demo
.foo {
  @include log('Message');
}

SassMeister-output.css

Message