IonSchemaSystemImpl.loadSchema's execption handling obfuscates cause of exceptions

[dlurton]

This code here is suboptimal for a number of reasons:

• If an exception occurs during the loading of any schema, the cause of the exception is lost--instead the cause should be included as the second argument of IonSchemaException so that the stack trace is plainly visible to those who need to troubeshoot. As it is now, the troubleshooter has to go thru a lot of extra hoops to diagnose failed schema loading.
• This pattern seems to suggest it's ok to throw exceptions when a schema can't be loaded, i.e. during normal operation. I.e. that an Authority should just throw an exception and the Ion schema system will continue searching the other loaded authorities for the schema, however:
  • Throwing exceptions can degrade performance.
  • It is generally never a good idea to throw exceptions during normal program execution--they should only be used for unrecoverable and unexpected errors.

This code really should be refactored to ameliorate these issues.