Improve errors and warnings for content read from standard input

[jhradilek]

When reading from standard input, the dita-convert utlity previously displayed the exposed IO object in error messages and warnings:

$ cat file.dita | dita-convert -g
dita-convert: <_io.TextIOWrapper name='<stdin>' mode='r' encoding='utf-8'>: error: unsupported outputclass "snippet", use -t/--type

This pull requests excludes the file name from error messages and warnings when reading from standard input:

$ cat file.dita | dita-convert -g
dita-convert: error: unsupported outputclass "snippet", use -t/--type

Implementation checklist

• The code changes come with the corresponding test cases
• The code changes pass all tests (run python3 -m unittest in the project directory)
• The code changes come with appropriate documentation