is
Returns true when this exception's message smells like a TLS / X.509 certificate validation failure.
The check is deliberately string-based on the exception message — Krill's shared module compiles for JVM, Android, iOS, and wasmJs, and each platform raises a different concrete exception type for the same underlying TLS problem. Matching "signature" and "certification" covers the messages produced by the JDK (SunCertPathBuilderException, SignatureException), Android (CertPathValidatorException), Darwin's secure transport, and the browser's fetch failure surface.
Callers (notably NodeHttp) use this to decide whether to evict a stored peer certificate and trigger a fresh trust handshake before retrying.