Webmachine/Cowboy error pretty-printing

Webmachine errors go from:

```
2013-02-27 22:03:25.813 [error] <0.15064.1391> webmachine error:
                     path="/buckets
{error,{error,{badmatch,{error,timeout}},[{riak_kv_wm_buckets,produce_bucket_list,2,[{file,"src/riak_kv_wm_buckets.erl"},{line,123}]},{webmachine_resource,resource_call,3,[{file,"src/webmachine_resource.erl"},{line,169}]},{webmachine_resource,do,3,[{file,"src/webmachine_resource.erl"},{line,128}]},{webmachine_decision_core,resource_call,1,[{file,"src/webmachine_decision_core.erl"},{line,48}]},{webmachine_decision_core,decision,1,[{file,"src
```

To:
```
23:42:51.632 [error] Webmachine error at path "/buckets" : no match of right hand value {error,timeout} in riak_kv_wm_buckets:produce_bucket_list/2 line 123
```

There's also similar rewriting done for cowboy 0.8+ errors.