Mutagen using less reliable `err == <sentinal>` rather than `errors.Is(err,<sentinal>)`

[mikeschinkel]

While trying to fix Mutagen to allow me to use it for my use-case, my IDE and linter kept complaining about err == <sentinal> when AFAIK errors.Is(err,<sentinal>) [has been recommended since it was added in Go 1.13.

In order to shut up my linter I went ahead and found all the cases in this repo where err == <sentinal> and changed them to be errors.Is(err,<sentinal>).

I also found one of linter concern where it was flagging shortLastBlock might be nil because of an indirect check via a haveShortLastBlock variable. My PR replaces that variable with a simple shortLastBlock!=nil since there was only one case and that stopped the linter from flagging it.