error: add is_fatal helper

This commit adds a method to `Error` for testing whether an error should
be considered fatal, e.g. should stop any further path building
progress.

Having this in a central place means we can avoid duplicating the match
arms in multiple places, where they are likely to fall out-of-sync.

src/error.rs

@@ -257,6 +257,18 @@ impl Error {
             Error::UnknownIssuer => 0,
         }
     }
+
+    /// Returns true for errors that should be considered fatal during path building. Errors of
+    /// this class should halt any further path building and be returned immediately.
+    #[inline]
+    pub(crate) fn is_fatal(&self) -> bool {
+        matches!(
+            self,
+            Error::MaximumSignatureChecksExceeded
+                | Error::MaximumPathBuildCallsExceeded
+                | Error::MaximumNameConstraintComparisonsExceeded
+        )
+    }
 }
 
 impl fmt::Display for Error {