Don't swallow exceptions on file not found, unless in interactive mode.

When visclaw attempts to load a solution frame and the file is not found, it will crash and return a traceback -- unless it is in interactive mode, in which case it will warn the user that it is swallowing an IOError exception.

This will make it much easier to debug other issues affecting loading of a solution frame.  Currently, such debugging is difficult because the errors are swallowed.

Also: clean up a few issues found by pylint.