cmd/present better error message

[packrat386]

Provide a better suggestion for how to fix the error when static files cannot be loaded due to installing and running in module aware mode.

See: https://github.com/golang/go/issues/43459

[gopherbot]

This PR (HEAD: bee1ab494bec3ba8710a2a178a05deae21df5a8f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tools/+/327129 to see it.

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