[Feature] Normalize `unknown` error handling in smoke utilities

[EmmaYuan1015]

• The errorToString and asError utility functions have been added to specifically handle unknown error types.
• All catch (err) related code uses type guards and asError/errorToString to avoid direct access to properties.
• When functions like execSync and mkdirp throw and log errors, they are standardized using errorToString.
• The smokeTestFail call already passes the errorToString(err) result, ensuring type safety.