cmd/go: unuseful error message when solving dependencies

[rakyll]

The build is failing with "error loading module requirements" without any insights on why module requirements didn't load. It also doesn't give any helpful details whether it was due to a third party library or not.

$ go run .
go: finding github.com/GoogleCloudPlatform/cloudsql-proxy/proxy/dialers/postgres latest
go: finding github.com/GoogleCloudPlatform/cloudsql-proxy/proxy/dialers latest
go: finding github.com/GoogleCloudPlatform/cloudsql-proxy/proxy latest
go: finding github.com/GoogleCloudPlatform/cloudsql-proxy latest
go: finding golang.org/x/oauth2/google latest
go: finding golang.org/x/oauth2 latest
go: finding golang.org/x/net/context latest
go: finding golang.org/x/net latest
go: finding cloud.google.com/go v0.34.0
go: finding golang.org/x/net v0.0.0-20180724234803-3673e40ba225
go: cloud.google.com/go@v0.34.0: unknown revision v0.34.0
go: finding github.com/golang/protobuf v1.2.0
go: finding golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a
go: error loading module requirements

[jayconrod]

It looks like the error was buried in the rest of the chatter:

go: cloud.google.com/go@v0.34.0: unknown revision v0.34.0

This is related to #30661, but not quite a dup. But CL 166984 should fix this when it finally gets submitted.

[bcmills]

This seems like a dup of #26152: one of the lines was useful, but it was buried in less-useful chatter.