Fixes incompatible types warning

[bmartin2015]

A quick fix to clear up the following warnings:

warning: incompatible types:

    %ExMarshal.Errors.EncodeError{} !~ atom()

in expression:

    # lib/ex_marshal/errors/encode_error.ex:5
    exception.reason()

where "exception" was given the type %ExMarshal.Errors.EncodeError{} in:

    # lib/ex_marshal/errors/encode_error.ex:4
    %ExMarshal.Errors.EncodeError{} = exception

where "exception" was given the type atom() (due to calling var.fun()) in:

    # lib/ex_marshal/errors/encode_error.ex:5
    exception.reason()

HINT: "var.field" (without parentheses) implies "var" is a map() while "var.fun()" (with parentheses) implies "var" is an atom()

Conflict found at
  lib/ex_marshal/errors/encode_error.ex:5: ExMarshal.Errors.EncodeError.message/1

warning: incompatible types:

    %ExMarshal.Errors.DecodeError{} !~ atom()

in expression:

    # lib/ex_marshal/errors/decode_error.ex:5
    exception.reason()

where "exception" was given the type %ExMarshal.Errors.DecodeError{} in:

    # lib/ex_marshal/errors/decode_error.ex:4
    %ExMarshal.Errors.DecodeError{} = exception

where "exception" was given the type atom() (due to calling var.fun()) in:

    # lib/ex_marshal/errors/decode_error.ex:5
    exception.reason()

HINT: "var.field" (without parentheses) implies "var" is a map() while "var.fun()" (with parentheses) implies "var" is an atom()

Conflict found at
  lib/ex_marshal/errors/decode_error.ex:5: ExMarshal.Errors.DecodeError.message/1

[bmartin2015]

@gaynetdinov Any chance you could review this please?