don't use Markdown-style backtick in error msgs

Because backtick has a different meaning in the shell, and the user
may copy-and-paste such text.

Some light re-wording included as well, and a reference to "brew cask edit"
changed to "brew cask cat" for simplicity.
Merge pull request #4970 from rolandwalker/no_backtick_in_error_messages

don't use Markdown-style backtick in error msgs