Improve error message when formatting.

[marcomorain]

Hi there, thanks for making this extension!

When I format a file in a project without cljfmt, I get the following error:

FileNotFoundException Could not locate cljfmt/coreinit.class or cljfmt/core.clj on classpath. clojure.lang.RT.load (RT.java:456)

It would be awesome if the extension could catch this error and tell the user what they need to do to get code formatting to work.

It would be great to get the same information added to the README, too.

Thanks

[avli]

README was updated and now contains the information about the manual configuration.