Add Velocity tool to manipulate exceptions

It would be done using Commons Lang's ExceptionUtils class:
http://commons.apache.org/proper/commons-lang/javadocs/api-3.1/org/apache/commons/lang3/exception/ExceptionUtils.html

Use case:

• script services should never throw exceptions
• instead they should return null (or empty string, etc)
• they should store the exception in the execution context and provide an API to access the exception
• it's up to the caller (*.vm files, wiki pages) to decide what to do when there's an error.
• In some *.vm files we need to display the error and being able to use $exceptiontool.getStackTrace($myscriptservice.lastError) would be nice. Otherwise it forces all script services to offer an API to transform an Exception into a String. In addition ExceptionUtils offers other interesting methods, such as getting the root cause, etc.