Improved error messages for failures

[hlverstoep]

When a build fails we get error messages like these:

Error when running Shake build system:
  at want, called at LOCATION
* Depends on: RULE
  at need, called at LOCATION
  at error, called at LOCATION
* Raised the exception:
Error, file does not exist and no rule available:
  FILE

However, if you have a lot of rules it is not always obvious where to start looking. Would it be possible to add more location information?

Error when running Shake build system:
  at want, called at LOCATION
* Depends on: RULE, defined at LOCATION
  at need, called at LOCATION
  at error, called at LOCATION
* Raised the exception:
Error, file does not exist and no rule available:
  FILE