Improve error reporting from Kernel scripts

* Ensure that we have a .dill file and it has the correct version before
  we start decoding it in dump.dart.

* Throw errors, not strings.  Ensure that they have a useful
  toString().

* Do not print usage except when the command is invoked
  incorrectly (wrong number of arguments).  Once we get into the
  command it's less likely that it's been invoked incorrectly and more
  likely that something else has gone wrong.

  Because these utilities are invoked from other scripts (like the
  fasta command), printing their usage for problems other than
  invoking them correctly doesn't match the way that they were invoked
  and it's confusing.

Change-Id: I7832383594d2b3719a0a7a7392ba4685717a79d2
Reviewed-on: https://dart-review.googlesource.com/c/78206
Commit-Queue: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
9 files changed
tree: 5813b46f193b071d3e50fd9759e6fcf860e257e1
  1. .github/
  2. build/
  3. client/
  4. docs/
  5. pkg/
  6. runtime/
  7. samples/
  8. samples-dev/
  9. sdk/
  10. tests/
  11. third_party/
  12. tools/
  13. utils/
  14. .clang-format
  15. .gitattributes
  16. .gitconfig
  17. .gitignore
  18. .gn
  19. .mailmap
  20. .packages
  21. AUTHORS
  22. BUILD.gn
  23. CHANGELOG.md
  24. codereview.settings
  25. CONTRIBUTING.md
  26. dartdoc_options.yaml
  27. DEPS
  28. LICENSE
  29. PATENTS
  30. PRESUBMIT.py
  31. README.dart-sdk
  32. README.md
  33. WATCHLISTS
README.md

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