Clarify that rescue_from must return a Rack::Response, re-raise or ca…

…ll error./script/quick-start Closes #1151. [ci-skip]

README.md

@@ -1529,8 +1529,7 @@ class Twitter::API < Grape::API
 end
 ```
 
-You can rescue all exceptions with a code block. The `error!` wrapper
-automatically sets the default error code and content-type.
+You can rescue all exceptions with a code block. The `error!` wrapper automatically sets the default error code and content-type.
 
 ```ruby
 class Twitter::API < Grape::API
@@ -1551,9 +1550,7 @@ class Twitter::API < Grape::API
 end
 ```
 
-
-You can also rescue specific exceptions with a code block and handle the Rack
-response at the lowest level.
+You can also rescue specific exceptions with a code block and handle the Rack response at the lowest level.
 
 ```ruby
 class Twitter::API < Grape::API
@@ -1612,6 +1609,8 @@ rescue_from RuntimeError, rescue_subclasses: false do |e|
 end
 ```
 
+The `rescue_from` block must return a `Rack::Response` object, call `error!` or re-raise an exception.
+
 #### Unrescuable Exceptions
 
 `Grape::Exceptions::InvalidVersionHeader`, which is raised when the version in the request header doesn't match the currently evaluated version for the endpoint, will _never_ be rescued from a `rescue_from` block (even a `rescue_from :all`) This is because Grape relies on Rack to catch that error and try the next versioned-route for cases where there exist identical Grape endpoints with different versions.