imports: show detailed error when directory cannot be opened
imports: show detailed error when directory cannot be opened

Fixes golang/go#24154

Change-Id: I2834cc2578e911499cbce5412fe6a9041248794d
Reviewed-on: https://go-review.googlesource.com/97359
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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