Do not raise error in save_artifacts function

The error has been replaced with a log entry (if the copying process
failed, the current test will be marked as failed). Also, the PID of
the server process was removed from the log string because the process
may be missing.

Close #321

Author: Oleg Chaplashkin

luatest/server.lua

@@ -353,8 +353,8 @@ end
 function Server:save_artifacts()
     local ok, err = fio.copytree(self.workdir, self.artifacts)
     if not ok then
-        error(('Failed to copy artifacts for server (alias: %s, workdir: %s, pid: %d): %s')
-            :format(self.alias, fio.basename(self.workdir), self.process.pid, err))
+        log.error(('Failed to copy artifacts for server (alias: %s, workdir: %s): %s')
+            :format(self.alias, fio.basename(self.workdir), err))
     end
 end