Improve error message reported to end user

spec/ameba/severity_spec.cr

@@ -34,7 +34,7 @@ module Ameba
       end
 
       it "raises when name is incorrect" do
-        expect_raises(Exception, "Incorrect severity name BadName. Try one of available names") do
+        expect_raises(Exception, "Incorrect severity name BadName. Try one of Error, Warning, Convention") do
           Severity.parse("BadName")
         end
       end

src/ameba/severity.cr

@@ -41,7 +41,7 @@ module Ameba
     def self.parse(name : String)
       super name
     rescue ArgumentError
-      raise "Incorrect severity name #{name}. Try one of available names"
+      raise "Incorrect severity name #{name}. Try one of #{values.map(&.to_s).join(", ")}"
     end
   end