Better configuration error

[st0012]

Hello, I noticed that if we configure a not yet registered uploader like:

configure.storage = :foo

It will raise

TypeError: no implicit conversion of nil into String

which is caused by eval in this line

And this is a very confusing message, you have to dive into source code to find out what's wrong. So I wonder if there anyone think raise a custom exception here will be a good idea?

[thomasfedb]

@st0012 Are you able to provide a pull request to improve the error message?

[st0012]

@thomasfedb Sure, I will work on it.

[bensie]

Closed by #1779