Make exception string representations also the message

[danielsdeleo]

In ChefDK, we assume that SomeException#message will be informative, but the exception classes here assume you will be calling SomeException#to_s. By assigning to_s to message, we make everyone happy.

This is a partial fix for chef-boneyard/chef-dk#564 (need to ship, bump version constraint and apply a small fix to chef-dk for a similar error case)

[danielsdeleo]

BTW, when pulling this patch into chef-dk, when I follow the repro for chef-dk 546, I see:

Building policy example-application-service
Expanded run list: recipe[foo::default]
Caching Cookbooks...
Installing foo >= 0.0.0 from path
Error: Failed to generate Policyfile.lock
Reason: (CookbookOmnifetch::NotACookbook) The resource at '/Users/ddeleo/oc/chef-dk/cookbooks/example_cookbook' does not appear to be a valid cookbook. Does it have a metadata.rb?

[ksubrama]

👍

[tyler-ball]

👍