Return full stack trace on unexpected errors

[gaul]

In #352 a user reported an error with this HTTP error:

<h2>HTTP ERROR: 400</h2>
<p>Problem accessing /. Reason:
<pre>    com.google.common.io.BaseEncoding$DecodingException: Unrecognized character: .</pre></p>

S3Proxy should return the full stack trace to make debugging easier.

[kortemik]

I think this should be configureable