Nilerr Versions Save

• Update modules (Thanks @masa213f)
• Update go.mod go version to 1.15

#4 by @PeterIvanov (Thanks!! 🎉 )

• if err is used as an argument in a function call, it is not considered a mistake to return nil
• such uses with .Error() and with non-err or vararg parameters are also supported
• a line number is added to waning text, because in some cases the offending error assignment is not that easy to find
• multiple possible assignments are supported - for example, if err is checked and then overwritten in a loop
• after a negative err check, warning is not issued if there are other conditions in if statement
• added support for multiple error return values