Support findbugsFailOnError for report types other than Xml

[josephearl]

Currently only XML is supported because sbt-findbugs reads the XML report to see what bugs (and how many) have been found.

One option could be use the exit code or other just to tell whether FindBugs has been successful and remove any reporting.

Another option could be to always have FindBugs generate the XML format, then generate the HTML or other report type from the XML using XSL.

[josephearl]

I think the best option here is to always have FindBugs itself always generate the XML report. The only variable option that would be passed to FindBugs would be whether to generate with messages or not.

Output types other than XML would be achieved by adding XSL transformations.

The question now is whether the public API should change - should the report types other than XML be removed from the report type and instead implemented as factories or constants for the corresponding XSLT transformations?