Reword error when file doesn't exist

Applied suggestion from PR

Co-authored-by: Vitalii Elenhaupt <3624712+veelenga@users.noreply.github.com>

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: Unable to find config file foo.yml") do
+        expect_raises(Exception, "Config file is invalid: Config file does not exist foo.yml") do
           Config.load "foo.yml"
         end
       end

src/ameba/config.cr

@@ -120,7 +120,8 @@ class Ameba::Config
 
   protected def self.read_config(path = nil)
     if path
-      return File.exists?(path) ? File.read(path) : raise("Unable to find config file #{path}")
+      raise ArgumentError.new("Config file does not exist #{path}") unless File.exists?(path)
+      return File.read(path)
     end
     each_config_path do |config_path|
       return File.read(config_path) if File.exists?(config_path)