Assorted typos and docstring mismatches #542

Small issues, collected in one place:

• BibtexFormat.indent docstring says "Default: single space", but the default is a tab (bibtexparser/writer.py:135, :143)
• BibtexFormat.value_column docstring claims it affects String blocks, but _treat_string never aligns (writer.py:41-48, :154)
• _val_intent_string — typo for "indent" (writer.py:35)
• _BlockJunk is presumably meant to be "chunk"; "junk" also appears in a user-facing error message (bibtexparser/middlewares/sorting_blocks.py:24, :36)
• Block.__init__ assigns _parser_metadata twice (bibtexparser/model.py:34-36)
• Missing spaces in concatenated message strings: bibtexparser/middlewares/interpolate.py:49-52, bibtexparser/middlewares/enclosing.py:133, :166, bibtexparser/exceptions.py:53
• "Exception where a invalid bibtex file" (exceptions.py:22)