eth/downloader: TestBoundedXXXForkedSyncXXXFast tests are flaky on Travis

[adamschmideg]

TestBoundedHeavyForkedSync63Fast: https://travis-ci.org/github/ethereum/go-ethereum/jobs/669570148
TestBoundedHeavyForkedSync64Fast: https://travis-ci.org/github/ethereum/go-ethereum/jobs/668816462
TestBoundedForkedSync63Fast: https://travis-ci.org/github/ethereum/go-ethereum/jobs/669527255
TestBoundedForkedSync64Fast: https://travis-ci.org/github/ethereum/go-ethereum/jobs/669534995

[holiman]

Yeah, this is exactly one of those cases where we'd be better off with go 1.13-type error handling, so we wouldn't reuse the same error in N different places without any context information

[adamschmideg]

This is the first commit where these tests started to fail: 1b6b0df