Fix mangled Protobuf.EncodeError

[v0idpwn]

Because of the recursive implementation of encode_fields, unrelated
fields would get mixed up in encode errors.

For example, our struct %TestMsg.Foo has fields :a, :b, :c,
:d, e (and more...). If we had an error at :e, we would get the
error popping up for all fields coming before it:

iex(1)>  %TestMsg.Foo{a: 90, e: %TestMsg.Foo.Bar{a: "not_a_number"}} |> Protobuf.encode()
** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo#a: ** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo#b: ** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo#c: ** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo#d: ** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo#e: ** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo.Bar#a: ** (Protobuf.EncodeError) "not_a_number" is invalid for type :int32
    (protobuf 0.13.0) lib/protobuf/encoder.ex:71: Protobuf.Encoder.encode_fields/5
    (protobuf 0.13.0) lib/protobuf/encoder.ex:33: Protobuf.Encoder.encode_with_message_props/2
    (protobuf 0.13.0) lib/protobuf/encoder.ex:18: Protobuf.Encoder.encode/1
    iex:1: (file)

After the patch, only the relevant fields are mentioned in the error:

iex(3)> %TestMsg.Foo{a: 90, e: %TestMsg.Foo.Bar{a: "not_a_number"}} |> Protobuf.encode()
** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo#e: ** (Protobuf.EncodeError) Got error when encoding TestMsg.Foo.Bar#a: ** (Protobuf.EncodeError) "not_a_number" is invalid for type :int32
    (protobuf 0.13.0) lib/protobuf/encoder.ex:75: Protobuf.Encoder.encode_field/4
    (protobuf 0.13.0) lib/protobuf/encoder.ex:63: Protobuf.Encoder.encode_fields/5
    (protobuf 0.13.0) lib/protobuf/encoder.ex:33: Protobuf.Encoder.encode_with_message_props/2
    (protobuf 0.13.0) lib/protobuf/encoder.ex:18: Protobuf.Encoder.encode/1
    iex:3: (file)