Allow exit codes to be deduced from Exceptions

[philwebb]

As part of the spring-task work it would be nice if specific exit codes could be generated by exceptions.

Currently exit code generation looks like this:

        System.exit(SpringApplication
                .exit(SpringApplication.run(SampleBatchApplication.class, args)));

It would be nice if you could just throw an exception from a command line runner and if it implements ExitCodeGenerator have the appropriate code returned.

[philwebb]

/cc @mminella @cppwfs

[philwebb]

We need a way to also map exceptions without needing then to implement the interface.

[wilkinsona]

Reopening as I'm seeing an NPE on master:

java.lang.NullPointerException: null
    at org.springframework.boot.SpringApplication.getExitCodeFromMappedException(SpringApplication.java:886) [classes/:na]
    at org.springframework.boot.SpringApplication.getExitCodeFromException(SpringApplication.java:875) [classes/:na]
    at org.springframework.boot.SpringApplication.handeExitCode(SpringApplication.java:861) [classes/:na]
    at org.springframework.boot.SpringApplication.handleRunFailure(SpringApplication.java:832) [classes/:na]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:318) [classes/:na]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1180) [classes/:na]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1169) [classes/:na]
    at sample.data.mongo.SampleMongoApplication.main(SampleMongoApplication.java:59) [classes/:na]

I think it's because there's no mapper in the context and SpringApplication tries to call it anyway