Handle missing exchange rate provider, allow fallback for missing rates (#955)

* Clean up exchange rate logic

* Remove stale method

test/models/provider/synth_test.rb

@@ -1,9 +1,26 @@
 require "test_helper"
+require "ostruct"
 
 class Provider::SynthTest < ActiveSupport::TestCase
   include ExchangeRateProviderInterfaceTest
 
   setup do
-    @subject = Provider::Synth.new
+    @subject = @synth = Provider::Synth.new("fookey")
+  end
+
+  test "retries then provides failed response" do
+    Faraday.expects(:get).returns(OpenStruct.new(success?: false)).times(3)
+
+    response = @synth.fetch_exchange_rate from: "USD", to: "MXN", date: Date.current
+
+    assert_match "Failed to fetch exchange rate from Provider::Synth", response.error.message
+  end
+
+  test "retrying, then raising on network error" do
+    Faraday.expects(:get).raises(Faraday::TimeoutError).times(3)
+
+    assert_raises Faraday::TimeoutError do
+      @synth.fetch_exchange_rate from: "USD", to: "MXN", date: Date.current
+    end
   end
 end