Two minor changes

[Artoria2e5]

This PR fixes two very minor headaches I had when playing with pdf2txt.py.

• A wrong -t value produces a cryptic message. Now it raises a proper ValueError.
• Asking for -t tag produces a runtime error. Now it runs correctly (as in, it successfully produces incomprehensible garbage that hasn't been run through layout analysis:))

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
  • Do I really want to test that Tag thing? Do I? Please don't hurt me
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[Artoria2e5]

Looks like I am being attacked for the f-string by an outdated flake8 or something. (I will fix the whitespace ASAP, but I am currently in a bit of rebase hell of my own making.)

[pietermarsman]

I can see why the UnboundLocalError is not very helpful.

I can also reproduce the TypeError when extracting tags.

Will cleanup both a bit before merging.

[pietermarsman]

@Artoria2e5 Thanks for noticing and fixing