[Process] Warn user with a useful message when tmpfile() failed
merged branch Seldaek/proctemp (PR #7066)

This PR was merged into the 2.1 branch.

Commits
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5e8d844 [Process] Warn user with a useful message when tmpfile() failed

Discussion
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[Process] Warn user with a useful message when tmpfile() failed

Simple warning so users know what's going on instead of getting a proc_open error, see composer/composer#1581.

Can't get the PR template since ~~symfony.com~~ my dns resolver is dead :)