Fix foreign account sync crash (#794)

* Fix foreign account sync crash

* Refactor synth provider and show UI error if not configured

* Generate error message on missing exchange rates while converting balances

* Ignore sync messaged in i18n-tasks unused

* Generate missing exchange rate error during entry normalization

* Update alert classes

app/models/provider/synth.rb

@@ -5,6 +5,10 @@ class Provider::Synth
     @api_key = api_key || ENV["SYNTH_API_KEY"]
   end
 
+  def configured?
+    @api_key.present?
+  end
+
   def fetch_exchange_rate(from:, to:, date:)
     retrying Provider::Base.known_transient_errors do |on_last_attempt|
       response = Faraday.get("#{base_url}/rates/historical") do |req|