hdf5 export - filename requires .h5 extension - should be added automatically, or better error message

[k-dominik]

Currently, when exporting to hdf5, the users are required to give the filename an .h5 extension - without this being clear in the UI somewhere. When a user fails to do so, the only information they get is some java backtrace:

[ERROR] Command errored: Export HDF5
java.lang.IllegalArgumentException: HDF5 export file must have '.h5' suffix
	at org.ilastik.ilastik4ij.io.ExportCommand.run(ExportCommand.java:85)
	at org.scijava.command.CommandModule.run(CommandModule.java:196)
	at org.scijava.module.ModuleRunner.run(ModuleRunner.java:165)
	at org.scijava.module.ModuleRunner.call(ModuleRunner.java:125)
	at org.scijava.module.ModuleRunner.call(ModuleRunner.java:64)
	at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:247)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:750)

Also, the UI there is a file picker. Usually, the file does not exist yet, this also needs to be changed.