[vm] Elaborate on DartDev missing error.
[vm] Elaborate on DartDev missing error.

This change refactors CreateAndSetupDartDevIsolate slightly to
exit as soon as we know we cannot resolve DartDev.
Expands on the error message to be a bit more informative about
what went wrong.

Before:
  Failed to start the Dart CLI isolate
  (null).
  Observatory server failed to start after 1 tries
  [...]

After:
  Failed to start the Dart CLI isolate. Could not resolve DartDev snapshot or kernel.
  [...]

TEST=CQ

Change-Id: Iad3302dc4e64eef06b97c8b76a82b04c97a7089e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/220546
Commit-Queue: Clement Skau <495593248b6158163719b90dde8357b589c8e5d0@google.com>
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk