Replace err== w/errors.Is()

[mikeschinkel]

This PR replaces all err== w/errors.Is() in this repo's codebase — as recommended for Go 1.13+ — with more details at #504.

Also, I omitted haveShortLastBlock as using it caused lint to flag otherwise correct use of shortLastBlock as potentially being nil. I just used shortLastBlock!=nil instead which made my linter happy.

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I understand you may not want to accept these changes, and if so, I respect that. This less fragile error handling can be infinitesimally slower, and/or you may prefer not to remove haveShortLastBlock.

I made these changes for my own to make it easier to work on my fork for #505. I would be very happy if you allow me to give these back to the community, but if you dislike them I will just maintain my fork as long as #505 is not accepted and/or my use-case for #505 is not otherwise resolved, and as long as I am using Mutagen to update Docker volumes on a remote machine.

[mikeschinkel]

Sorry for the multiple force-pushes. This was the first time I had to create a signed commit and it took a few tries to get it all working.

[rfay]

Related:

• https://tpaschalis.me/golang-multierr/