search: update error message for structural query alert

Created by: leonore

found as part of this slack thread on #feedback-dogfood that the alert for a structural search does not make much sense in an insights context as we don't have a UI toggle for patterntype:structural.

this updates the error message so that it might be a bit more useful in those cases

Test plan

also pushed to a backend-integration branch.

validated the new error in an insight locally:

using the query: <Tooltip...content...>...</Tooltip>