Show informative error messages by default (`show_error=True`)

A frustrating experience on Spaces is running a prediction that just fails with a generic "Error" and no details. Consider making show_error=True the default so users get an informative message.

Tasks

• Audit the ~100 most popular Spaces to check whether showing exception messages by default would leak sensitive information
• Based on that, decide whether to change the default, or to show a sanitized message (exception type and message, no traceback)
• Update docs to explain the behavior and how to opt out

Related: #11149 (better UX around errors)