Handle connection timeout in the publish functions

[ppikula]

Hey, I noticed during the tests that the connection timeout error is not handled in the lib. My stack trace below. I'd like to be able to handle it in my app's logic.

 ** (WithClauseError) no with clause matching: {:error, :timeout}
 code: conn = ...
 stacktrace:
   (tortoise 0.9.5) lib/tortoise.ex:269: Tortoise.publish/4

[coveralls]

Pull Request Test Coverage Report for Build 1

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 88.043%

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Totals
Change from base Build 51:	0.2%
Covered Lines:	567
Relevant Lines:	644

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💛 - Coveralls