MooseException
MooseException is a normal C++ derived "exception" object. It's purpose is to give developers an opportunity to terminate a solve, while informing MOOSE that it should perform the necessary steps to clean up the current stack, notify other threads/processors of the error and communicate with the solver that the current solve has failed. When applicable and possible, the solver will allow MOOSE to cut the time step and make another attempt at a solve.
This exception should be used directly in user code when non-fatal situations are encountered such as the inability to converge a local Newton solve in a material, or if a interpolated value from a variable or lookup table ends up out-of-range.
MOOSE is setup to catch MooseExceptions inside of several user defined callbacks that occur during the solve's computeResidual, and computeJacobian callbacks. Note that these callbacks also include the evaluation of several other objects such as Materials, Constraints, and occasionally objects such as Postprocessors (when requested). This makes it possible to try smaller solves from most user-defined objects within MOOSE.
How it works
MOOSE Wraps several threaded sections in PARALLEL_TRY/PARALLEL_CATCH macros (See SystemBase.h). PARALLEL_TRY does nothing (but provide scope). PARALLEL_CATCH invokes the FEProblem::checkExceptionAndStopSolve() (See FEProblemBase.h) routine after the threaded section has terminated and all threads on the same processor agree on the error status. checkExceptionAndStopSolve() then does parallel communication to ensure that all processors are notified of a single failure and cutback together.