cmd/cover: don't ignore os.Create error
cmd/cover: don't ignore os.Create error

Failing to create the output file would give confusing errors such as:

	cover: invalid argument

Also do out.Close() even if Execute() errored.

Fixes #17951.

Change-Id: I897e1d31f7996871c54fde7cb09614cafbf6c3fc
Reviewed-on: https://go-review.googlesource.com/33278
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,goodlang/good,derekmarcotte/go,Cofyc/go,suncycheng/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,linux-on-ibm-z/go,christopher-henderson/Go,derekmarcotte/go,codestation/go,josharian/go,polyverse-security/go,linux-on-ibm-z/go,Cofyc/go,codestation/go,Cofyc/go,linux-on-ibm-z/go,CAFxX/go,linux-on-ibm-z/go,polyverse-security/go,josharian/go,polyverse-security/go,codestation/go,Cofyc/go,polyverse-security/go,codestation/go,linux-on-ibm-z/go,suncycheng/go,golang/go,nawawi/go,josharian/go,vsekhar/elastic-go,CAFxX/go,josharian/go,suncycheng/go,ImJasonH/go,cloudflare/go,vsekhar/elastic-go,goodlang/good,codestation/go,goodlang/good,Cofyc/go,nawawi/go,polyverse-security/go,linux-on-ibm-z/go,derekmarcotte/go,derekmarcotte/go,nawawi/go,goodlang/good,golang/go,cloudflare/go,akutz/go,vsekhar/elastic-go,goodlang/good,codestation/go,golang/go,vsekhar/elastic-go,akutz/go,vsekhar/elastic-go,akutz/go,CAFxX/go,vsekhar/elastic-go,linux-on-ibm-z/go,ImJasonH/go,CAFxX/go,polyverse-security/go,golang/go,josharian/go,goodlang/good,nawawi/go,codestation/go,Cofyc/go,golang/go,ImJasonH/go,derekmarcotte/go,vsekhar/elastic-go,suncycheng/go,codestation/go,Cofyc/go,ImJasonH/go,ImJasonH/go,golang/go,goodlang/good,golang/go,goodlang/good,josharian/go,josharian/go,josharian/go,derekmarcotte/go,akutz/go,cloudflare/go,suncycheng/go,polyverse-security/go,cloudflare/go,akutz/go,derekmarcotte/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,Cofyc/go,vsekhar/elastic-go,suncycheng/go,polyverse-security/go,derekmarcotte/go,CAFxX/go,cloudflare/go,cloudflare/go,ImJasonH/go,suncycheng/go,ImJasonH/go,golang/go,suncycheng/go,linux-on-ibm-z/go,cloudflare/go,ImJasonH/go,akutz/go