database/sql: prevent ErrBadConn from driver.Open()

[tjenkinson]

because it seems this error is only meant to be internal, and if ErrBadConn is returned from here it can also be returned from the public api.

See some confusion about this here: go-sql-driver/mysql#1020

And a fix in the postgres driver here: lib/pq@04c77ed

If we already had this check a while back, then go-sql-driver/mysql#867 would never have been accepted and we'd have gone down the route of a custom dial function, preventing the concern now that users of https://github.com/go-sql-driver/mysql will start seeing higher error rates if the PR is reverted.

[gopherbot]

This PR (HEAD: cfda5c7) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/203580 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Gobot Gobot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/203580.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Daniel Theophanes:

Patch Set 1: Code-Review-2

I appreciate your time thinking about this and collecting multiple issues to
document your findings.

I'd be willing to accept this into some type of driver conformance test suite.
I don't want to put this in the sql package.

We should have a active sql driver test suite, and it is mostly my fault we don't.
I'm sorry.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/203580.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/203580.
After addressing review feedback, remember to publish your drafts!