fix(gnovm): improve error message for nil assignment in variable declaration #6294

Job	Run time
setup	7s
Run Main (gnodev) / Go Build / generated	13s
Run Main (gnodev) / Go Linter / lint	41s
Run Main (gnodev) / Go Test / test	45s
Run Main (gnofaucet) / Go Build / generated	12s
Run Main (gnofaucet) / Go Linter / lint	18s
Run Main (gnogenesis) / Go Build / generated	7s
Run Main (gnofaucet) / Go Test / test	14s
Run Main (gnogenesis) / Go Test / test	24s
Run Main (gnokeykc) / Go Build / generated	13s
Run Main (gnogenesis) / Go Linter / lint	20s
Run Main (gnokeykc) / Go Linter / lint	23s
Run Main (gnomd) / Go Build / generated	7s
Run Main (gnomd) / Go Linter / lint	23s
Run Main (gnokeykc) / Go Test / test	19s
Run Main (gnomd) / Go Test / test	29s
Run Main (gnomigrate) / Go Build / generated	6s
Run Main (gnomigrate) / Go Linter / lint	26s
Run Main (gnomigrate) / Go Test / test	22s
	6m 9s

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