Log error message in RecoveryHandler

[akrylysov]

The default behavior of the default Go http panic handler is to display the error message and the stack trace. When I use raven.RecoveryHandler it outputs only the stack trace, which makes debugging difficult.

Default behavior:

func handler(w http.ResponseWriter, r *http.Request) {
    var x []int
    x[1] = 1
}

func main() {
    http.HandleFunc("/", handler)
    http.ListenAndServe(":8080", nil)
}

2016/07/11 13:49:24 http: panic serving [::1]:50020: runtime error: index out of range
goroutine 25 [running]:
[stack trace]

With RecoveryHandler:

func main() {
    http.HandleFunc("/", raven.RecoveryHandler(handler))
    http.ListenAndServe(":8080", nil)
}

goroutine 21 [running]:
[stack trace]

This pull request makes RecoveryHandler output look like:

2016/07/11 14:19:41 runtime error: index out of range
goroutine 6 [running]:
[stack trace]

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This change is