Specify that it's Slack's credentials that are invalid

lib/slack_mattermost_emoji/slack_client.rb

@@ -32,7 +32,7 @@ module SlackMattermostEmoji
 
       begin
         @driver.find_element(css: 'p.alert.alert_error')
-        raise 'Invalid credentials'
+        raise 'Invalid Slack credentials'
       rescue Selenium::WebDriver::Error::NoSuchElementError
         nil
       end

spec/slack_client_spec.rb

@@ -31,7 +31,7 @@ RSpec.describe SlackMattermostEmoji::SlackClient do
       domain: ENV['SLACK_DOMAIN'],
       log: false,
     )
-    expect { client.authenticate }.to raise_error('Invalid credentials')
+    expect { client.authenticate }.to raise_error('Invalid Slack credentials')
   end
 
   it 'should download emojis from slack' do