v0.3.1

Fixed

• ioviewer: an output that cannot be written (a locked file, a read-only folder, a folder where the file should be or
  a file where its folder should be) ends the run with Error: Failed to export and exit code 3 for the page, the
  JSON output and the --json file alike. Before, only the page caught rejected data, and an I/O error ended the tool
  with an unhandled exception.
• ioviewer --serve: when the preview server cannot start (a port in use or out of range) or fails while serving, the
  tool exits with the new code 5 after printing the error; the output has been written. Before, it exited with 0, or 4
  for an engine error.
• JsonViewerExporter.Save serializes the data once before it opens the file, so data the serializer rejects (such as
  a non-finite coordinate) leaves an existing file as it was, as the HTML export already did; before, the file was
  emptied first. The file is still rewritten in place and keeps its permissions. The JSON outputs of ioviewer
  (-o *.json, --json) get this too, and its export error names the output that failed:
  Error: Failed to export '<input>' to '<output>': ....