Better error handling when scanning library tags

Better error handling when scanning library tags

Fixes Issue 516 
http://code.google.com/p/dart/issues/detail?id=516

#!/dart_bin
##library("foo");

void main() {
  print("goodbye");
}


Errors encountered while loading script: '/private/tmp/t.dart': Error: line 2 pos 2: Unrecognized library tag
##library("foo");
 ^
Committed: https://code.google.com/p/dart/source/detail?r=1664

[Ivan Posva, 2011-11-18 17:50:08]

LGTM -ip