encoding/gob: improve interface assignment error message
During the glob decoding process interface values are set to concrete
values after a test for assignability. If the assignability test fails
a slightly vague error message is produced. While technically accurate
the error message does not clearly describe the problem.
Rewrite the error message to include the usage of the word assignable,
which makes it clear the concrete value type is not assignable to the
interface value type.
Fixes #6467.
LGTM=r
R=golang-codereviews, rsc, r
CC=golang-codereviews
https://golang.org/cl/
71590043