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

For #16088.

Change-Id: I0ff480e95ef5af375be2ccc655f8b233a7bcd39d
Reviewed-on: https://go-review.googlesource.com/37755
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

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