Added support for logging Flow fault messages

[jongpie]

Closes #98 by adding new properties & updated logic in FlowLogEntry and FlowRecordLogEntry

• Both classes now have an optional string property faultMessage
• When the property faultMessage is set, the entry's logging level defaults to 'ERROR' (new behavior). When it's null, the default is still 'DEBUG' (existing behavior).
• When the property faultMessage is set, the field LogEntry__c.ExceptionType__c is automatically set to Flow.FaultError and the fault message is stored in the field LogEntry__c.ExceptionMessage__c - effectively, the Flow fault error is logged the same way an Exception is logged in the Apex methods