Add error message for grind script

Is this error message too long? T.T

review @devoncarew 

TEST=./grind  without setting DART_SDK, and DART_SDK/bin

RESULT=

sungguk@sungguk-R570-U-AFLGL:~/program_store/spark/ide$ ./grind 
Error: Unable to execute 'dart'
       Please set the DART_SDK environment variable to the SDK path.
       And add DART_SDK/bin to PATH environment variable.
       e.g.: 'export DART_SDK=your/path/to/dart/dart-sdk'
              'export PATH=$PATH:$DART_SDK/bin'

sungguk@sungguk-R570-U-AFLGL:~/program_store/spark/ide$