feat(core): add scheme error type

Sources/KiteCore/Errors.swift

@@ -0,0 +1,19 @@
+public struct SchemeError: Error, CustomStringConvertible {
+    public let message: String
+    public let irritants: [Value]
+
+    public init(_ message: String, irritants: [Value] = []) {
+        self.message = message
+        self.irritants = irritants
+    }
+
+    public var description: String {
+        if irritants.isEmpty { return message }
+        let parts = irritants.map { writeString($0) }.joined(separator: " ")
+        return "\(message) \(parts)"
+    }
+}
+
+func schemeError(_ message: String, _ irritants: [Value] = []) -> SchemeError {
+    SchemeError(message, irritants: irritants)
+}