Provide better error message when a linked dependency doesn't exist
Provide better error message when a linked dependency doesn't exist

This comes up for dart2js when the location for dart2js_platform.dill is invalid
or when the file was not provided. Today dart2js produces a bad crash message in
that scenario, instead of a readable error.

I wanted to provide a better message, but I'm not sure I can. For example, I
wanted to say "SDK Platform file not found", but I can't because linked
dependencies can be used for other puposes.

Change-Id: I1739ec36dab16b2fd72552e5e7716cd573a8ebd0
Reviewed-on: https://dart-review.googlesource.com/46943
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Peter von der Ahé <ad40f903ca639ca334d95dc5a5f0ae92bd708079@google.com>

dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk