Reword generic error message on config load

spec/ameba/config_spec.cr

@@ -85,7 +85,7 @@ module Ameba
       end
 
       it "raises when custom config file doesn't exist" do
-        expect_raises(Exception, "Config file is invalid: Config file does not exist foo.yml") do
+        expect_raises(Exception, "Unable to load config file: Config file does not exist foo.yml") do
           Config.load "foo.yml"
         end
       end

src/ameba/config.cr

@@ -115,7 +115,7 @@ class Ameba::Config
               end
     Config.new YAML.parse(content)
   rescue e
-    raise "Config file is invalid: #{e.message}"
+    raise "Unable to load config file: #{e.message}"
   end
 
   protected def self.read_config(path = nil)