Don't serialize server errors to client.

Server side errors may contain just about anything, such as e.g.
secrets, and, therefore, it seems like a bad idea to unconditionally
send these back to the client. In general, there is nothing a client can
do about an internal server error even if the specific internal error
message is known. The server developer can already see the error in
server logs so there isn't really any loss of information.

If the current behavior is actually desired it can be achieved by an
outer `try-catch` in the handler function. (Of course, an outer
`try-catch` can also be used to make sure that a server side error never
ends up at the client, but it is better to be safe by default.)

src/Servers.jl

@@ -467,7 +467,6 @@ function handle_connection(f, c::Connection, listener, readtimeout, access_log)
                 if isopen(http) && !iswritable(http)
                     request.response.status = 500
                     startwrite(http)
-                    write(http, sprint(showerror, e))
                     closewrite(http)
                 end
                 c.state = CLOSING

test/client.jl

@@ -611,7 +611,7 @@ end
         shouldfail[] = true
         seekstart(req_body)
         resp = HTTP.get("http://localhost:8080/retry"; body=req_body, response_stream=res_body, retry=false, status_exception=false)
-        @test String(take!(res_body)) == "500 unexpected error"
+        @test resp.status == 500
         # even if StatusError, we should still get the right response body
         shouldfail[] = true
         seekstart(req_body)
@@ -620,7 +620,6 @@ end
         catch e
             @test e isa HTTP.StatusError
             @test e.status == 500
-            @test String(take!(res_body)) == "500 unexpected error"
         end
         # don't throw a 500, but set status to status we don't retry by default
         shouldfail[] = false