Add SparkError class to handle error and exceptions.
rename errorString to errorcode.
fixed comments.
Merge remote-tracking branch 'origin/master' into add_error_handling
Fix error with renaming spark_error.dart file.
Add back tcp.dart
Rename spark_error.dart to spark_exception.dart.
nit fixed comment.
Merge pull request #1508 from dart-lang/add_error_handling

Add SparkError class to handle error and exceptions.