Core: improve build_release.py

Don't insist on log file removal until after usage is printed.

Some simple Python code improvements (x.find(y) != -1 --> y in x)

Make sure the git area is "clean" (has no unpushed changes, has pulled
all changes, has no untracked files)

Add label color detail when creating next github version label.

Closes #7913
release tool: fix 'log exists' check to not create the log before it checks ;)
release tool: must decode bytes to string