go/internal/gcimporter: return (possibly incomplete) package in case of error
go/internal/gcimporter: return (possibly incomplete) package in case of error

For #16088.

Change-Id: Ib38bda06a5c5d110ca86510043775c5cf229e6a8
Reviewed-on: https://go-review.googlesource.com/37756
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

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