Add Ruby snippet for defining and raising a custom error class

Author: ACR1209

snippets/ruby/error-handling/custom-error.md

@@ -0,0 +1,22 @@
+---
+title: Custom Error Class
+description: Defines and raises a custom error class in Ruby.
+author: ACR1209
+tags: ruby,error handling,custom error
+---
+
+```rb
+class MyCustomError < StandardError; end
+
+def risky_method(value)
+    raise MyCustomError, "Value must be positive" if value <= 0
+    "Valid value: #{value}"
+end
+
+# Usage:
+begin
+    puts risky_method(-1)
+rescue MyCustomError => e
+    puts e.message # Output: "Value must be positive"
+end
+```