doc: add error message (#691)

Author: KazuCocoa

lib/appium_lib/core/common/error.rb

@@ -3,9 +3,11 @@ module Core
     module Error
       class CoreError < StandardError; end
 
+      # Capability related errors
       class NoCapabilityError < CoreError; end
       class CapabilityStructureError < CoreError; end
 
+      # Appium related errors
       class NotSupportedAppiumServer < CoreError; end
       class NoSuchElementError < CoreError; end
 

lib/appium_lib/core/driver.rb

@@ -254,7 +254,9 @@ def validate_keys(opts)
         flatten_ops = flatten_hash_keys(opts)
 
         raise Error::NoCapabilityError unless opts.member?(:caps)
-        raise Error::CapabilityStructureError if !opts.member?(:appium_lib) && flatten_ops.member?(:appium_lib)
+        if !opts.member?(:appium_lib) && flatten_ops.member?(:appium_lib)
+          raise Error::CapabilityStructureError, 'Please check the value of appium_lib in the capability'
+        end
 
         true
       end