Handle missing input file explicitly

Check for and raise an exception when the user passes a
non-existing file as input. This makes such errors easier
to identify, as opposed to the code path and exception that
is raised with a non-existing file is passed (which also happens
to be the same exception raised when a non-XLSX file is passed,
making this error confusing).

xlsx2csv.py

@@ -1190,6 +1190,8 @@ def convert_recursive(path, sheetid, outfile, kwargs):
     try:
         if os.path.isdir(options.infile):
             convert_recursive(options.infile, sheetid, outfile, kwargs)
+        elif not os.path.exists(options.infile):
+            raise InvalidXlsxFileException("Input file not found!")
         else:
             xlsx2csv = Xlsx2csv(options.infile, **kwargs)
             if options.sheetname: