Set error log path

Forgot to set the path to the error log in #608.  Errors were only being
shown in console and not written to `logs/<Mix.env>/error.log`.

config/dev.exs

@@ -6,3 +6,5 @@ config :logger, :console, level: :info
 config :logger, :ecto,
   level: :debug,
   path: "logs/dev/ecto.log"
+
+config :logger, :error, path: "logs/dev/error.log"

config/prod.exs

@@ -7,3 +7,5 @@ config :logger, :console, level: :info
 config :logger, :ecto,
   level: :info,
   path: "logs/prod/ecto.log"
+
+config :logger, :error, path: "logs/prod/error.log"

config/test.exs

@@ -8,6 +8,8 @@ config :logger, :ecto,
   level: :warn,
   path: "logs/test/ecto.log"
 
+config :logger, :error, path: "logs/test/error.log"
+
 config :explorer, Explorer.ExchangeRates,
   source: Explorer.ExchangeRates.Source.NoOpSource,
   store: :none