Added ContinueOnError flag

[r10r]

Implements #73 .

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[r10r]

I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[r10r]

I've added my work email to github email settings.

[googlebot]

CLAs look good, thanks!

[hanwen]

can you follow instructions in CONTRIBUTING ?

I usually do code reviews in Gerrit.

thanks!

[r10r]

done!

Currently tests for the change are missing - I can add them, but this will
take some time.

Btw. one test is already failing in master (without my contribution):

root@shieldos-dev:~/gopath/src/github.com/google/zoekt/build# go test -v .
[...]
=== RUN   TestPartialSuccess
2019/01/07 15:08:30 finished /tmp/480465520/repo_v15.00000.zoekt: 3299 index bytes (overhead 2.9)
2019/01/07 15:08:30 finished /tmp/480465520/repo_v15.00001.zoekt: 3300 index bytes (overhead 2.9)
2019/01/07 15:08:30 finished /tmp/480465520/repo_v15.00002.zoekt: 3300 index bytes (overhead 2.9)
2019/01/07 15:08:30 finished /tmp/480465520/repo_v15.00003.zoekt: 3300 index bytes (overhead 2.9)
--- FAIL: TestPartialSuccess (0.01s)
    e2e_test.go:296: got shards [/tmp/480465520/repo_v15.00000.zoekt /tmp/480465520/repo_v15.00001.zoekt /tmp/480465520/repo_v15.00002.zoekt /tmp/480465520/repo_v15.00003.zoekt], want []