sky_snapshot should give a better error message for bad paths #122

diff --git a/sky/packages/sky/example/fitness/lib/main.dart b/sky/packages/sky/example/fitness/lib/main.dart
index ca7dfaf..c11a99e 100644
--- a/sky/packages/sky/example/fitness/lib/main.dart
+++ b/sky/packages/sky/example/fitness/lib/main.dart
@@ -8,6 +8,7 @@ import 'package:sky/editing/input.dart';
 import 'package:sky/painting/text_style.dart';
 import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/widgets.dart';
+import 'package:mojom/blah/blah.dart';

 part 'feed.dart';
 part 'fitness_item.dart';

FAILED: python ../../sky/tools/sky_snapshot.py out/Debug/sky_snapshot sky/packages/sky/example/fitness/lib/main.dart --package-root sky/packages/workbench/packages --snapshot out/Debug/gen/sky/packages/sky/example/fitness/fitness_snapshot.bin -C ../../
[0806/131321:FATAL:loader.cc(21)] Check failed: base::ReadFileToString(path, &source). sky/packages/workbench/packages/mojom/blah/blah.dart
#0 0x00000057304e base::debug::StackTrace::StackTrace()
#1 0x00000046e092 logging::LogMessage::~LogMessage()
#2 0x00000040ecb9 (anonymous namespace)::Fetch()
#3 0x00000040ea3d (anonymous namespace)::Loader::Import()
#4 0x00000040e591 HandleLibraryTag()
#5 0x0000007f2f30 dart::Parser::CallLibraryTagHandler()
#6 0x0000007f3b8c dart::Parser::ParseLibraryImportExport()
#7 0x0000007f4c42 dart::Parser::ParseLibraryDefinition()
#8 0x0000007c75d2 dart::Parser::ParseTopLevel()
#9 0x0000007c7315 dart::Parser::ParseCompilationUnit()
#10 0x0000006600c4 dart::Compiler::Compile()
#11 0x000000645b71 dart::CompileSource()
#12 0x000000645a83 Dart_LoadScript
#13 0x00000040eb7b LoadScript()
#14 0x000000411bae main
#15 0x7fbc4d9feec5 __libc_start_main
#16 0x00000040e32c <unknown>

Traceback (most recent call last):
  File "../../sky/tools/sky_snapshot.py", line 28, in <module>
    sys.exit(main())
  File "../../sky/tools/sky_snapshot.py", line 25, in main
    ], cwd=args.C)
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['out/Debug/sky_snapshot', 'sky/packages/sky/example/fitness/lib/main.dart', '--package-root=sky/packages/workbench/packages', '--snapshot=out/Debug/gen/sky/packages/sky/example/fitness/fitness_snapshot.bin']' returned non-zero exit status -6