Add check guide for faulty nodes in issues.md

[egplar]

Looks alright.

[klust]

Last sentecne: "Also, note that all errors are not due to problems with nodes as such, but might have to do with network issues." Maybe add something like: "And don't forget to question your code and the libraries that it uses. Out-of-memory messages are not always the result of system error. And segmentation violations are usually application or library errors that will not be solved by rebooting a node.."