blurb: Quitting blurb without saving the original message results in KeyError

[ericvsmith]

To reproduce in blurb 1.0.8: Run blurb. Exit the editor without saving any changes.

Result: a traceback with a KeyError:

$ blurb
Traceback (most recent call last):
File "/home/eric/venvs/py/bin/blurb", line 8, in
sys.exit(main())
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 1651, in main
sys.exit(fn(*filtered_args, **kwargs))
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 954, in add
blurb.load(tmp_path)
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 553, in load
self.parse(text, metadata=metadata, filename=filename)
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 543, in parse
finish_entry()
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 495, in finish_entry
throw("Blurb 'body' text must not be empty!")
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 486, in throw
raise BlurbError(f("Error in {filename}:{line_number}:\n{s}"))
File "/home/eric/venvs/py/lib/python3.8/site-packages/blurb.py", line 138, in f
return s.format_map(d)
KeyError: 'filename'

[ericvsmith]

This is a duplicate of #299. Closing.