title: How to reduce Seq[Either[A,B]] to Either[A,Seq[B]]?
body: <p>Given a sequence of eithers <code>Seq[Either[String,A]]</code> with <code>Left</code> being an error message. I want to obtain an <code>Either[String,Seq[A]]</code> where I get a <code>Right</code> (which will be a <code>Seq[A]</code>), if all elements of the sequence are <code>Right</code>. If there is at least one <code>Left</code> (an error message), I'd like to obtain the first error message or a concatenation of all error messages.</p>
  <p>Of course you can post cats or scalaz code but I'm also interested in code not using it.</p>
  <h2>Edit</h2>
  <p>I've changed the title, which originally asked for an <code>Either[Seq[A],Seq[B]]</code> to reflect the body of the message.</p>