env: add the same prefix to error messages to make it detectable by tests

env: add the same prefix to error messages to make it detectable by tests

The error message should start with `## Error: ` so that it's easily
detectable by tests without needing to have a complex regexp for
matching all possible error message patterns.

Let's add the `## Error: ` prefix to the error messages since it's the
one already in use.

Suggested-by: Stephen Warren <[email protected]>
Signed-off-by: Quentin Schulz <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Stephen Warren <[email protected]>
Tested-by: Stephen Warren <[email protected]>