internal/span: improve invalid uri message
internal/span: improve invalid uri message

Change-Id: Iaf4de4f3a3494868db130fe5fb6885fd28f2a4ea
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181684
Run-TryBot: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools