improving error message when invalid mailer configuration

[martinos]

If we don't configure properly the mailer, there is a compiler error that does not give much information.
See issue: #149

= Compilation error on file lib/mailer.ex ==
** (Protocol.UndefinedError) protocol Enumerable not implemented for nil
    (elixir) lib/enum.ex:1: Enumerable.impl_for!/1
    (elixir) lib/enum.ex:116: Enumerable.reduce/3
    (elixir) lib/enum.ex:1486: Enum.reduce/3
    lib/bamboo/mailer.ex:187: Bamboo.Mailer.parse_opts/2
    lib/mailer.ex:2: (module)
    (stdlib) erl_eval.erl:669: :erl_eval.do_apply/6
    (elixir) lib/kernel/parallel_compiler.ex:100: anonymous fn/4 in Kernel.ParallelCompiler.spawn_compilers/8

By doing the change in this pull request we get a better error message:

== Compilation error on file lib/mailer.ex ==
** (ArgumentError) application :basic_guardian is not loaded, or the configuration parameter BasicGuardian.Mailer is not set
    (elixir) lib/application.ex:203: Application.fetch_env!/2
    lib/bamboo/mailer.ex:187: Bamboo.Mailer.parse_opts/2
    lib/mailer.ex:2: (module)
    (stdlib) erl_eval.erl:669: :erl_eval.do_apply/6
    (elixir) lib/kernel/parallel_compiler.ex:100: anonymous fn/4 in Kernel.ParallelCompiler.spawn_compilers/8

[paulcsmith]

@martinos This is a great catch! Friendly error messages make things a lot nicer for people getting started. Can you rebase this branch and then I'll merge it in.

[martinos]

Done. Thanks

[paulcsmith]

Perfect. Thank you so much!