Lint and fix inconsistent style issues

[dandhlee]

Googlers see b/206698958.

Should also set up lint*

Few inconsistencies:

• Exception types returning non-specific exception types
• ' and " usage inconsistency
• based on https://google.github.io/styleguide/pyguide.html#310-strings, use "".join(string_array) rather than appending string using + or +=

and more should be fixed when refactoring.