Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

app/models/provider/base.rb

@@ -0,0 +1,18 @@
+
+class Provider::Base
+  ProviderError = Class.new(StandardError)
+
+  TRANSIENT_NETWORK_ERRORS = [
+    Faraday::TimeoutError,
+    Faraday::ConnectionFailed,
+    Faraday::SSLError,
+    Faraday::ClientError,
+    Faraday::ServerError
+  ]
+
+  class << self
+    def known_transient_errors
+      TRANSIENT_NETWORK_ERRORS + [ ProviderError ]
+    end
+  end
+end