Add Bamboo.ApiError.build_api_error

What changed?
=============

This commit adds a `Bamboo.ApiError.build_api_error` function that is a
counterpart to the `raise_api_error`. That way, adapters can use the
`build_api_error` function to return `{:error, %Bamboo.ApiError{}}`
tuples instead of raising errors.

lib/bamboo/api_error.ex

@@ -62,4 +62,28 @@ defmodule Bamboo.ApiError do
 
     raise(__MODULE__, message: message)
   end
+
+  def build_api_error(message), do: %__MODULE__{message: message}
+
+  def build_api_error(service_name, response, params, extra_message \\ "") do
+    message = """
+    There was a problem sending the email through the #{service_name} API.
+
+    Here is the response:
+
+    #{inspect(response, limit: 150)}
+
+    Here are the params we sent:
+
+    #{inspect(params, limit: 150)}
+    """
+
+    message =
+      case extra_message do
+        "" -> message
+        em -> message <> "\n#{em}\n"
+      end
+
+    %__MODULE__{message: message}
+  end
 end