chore: better unknown import error message

[leohhhn]

Description

Closes: #1871

This PR adds a suggestion to check the gno.mod file in case of an unknown import error. Most of the time the error happens because gno.mod is not tidy.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gnovm/pkg/gnolang/preprocess.go	0.00%	1 Missing ⚠️

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