support ajv-errors for scaffolder validation

Signed-off-by: Matt Benson <gudnabrsam@gmail.com>

docs/features/software-templates/input-examples.md

@@ -26,6 +26,25 @@ parameters:
         ui:help: 'Hint: additional description...'
 ```
 
+#### Custom validation error message
+
+```yaml
+parameters:
+  - title: Fill in some steps
+    properties:
+      name:
+        title: Simple text input
+        type: string
+        description: Description about input
+        maxLength: 8
+        pattern: '^([a-zA-Z][a-zA-Z0-9]*)(-[a-zA-Z0-9]+)*$'
+        ui:autofocus: true
+        ui:help: 'Hint: additional description...'
+    errorMessage:
+      properties:
+        name: '1-8 alphanumeric tokens (first starts with letter) delimited by -'
+```
+
 ### Multi line text input
 
 ```yaml

docs/features/software-templates/writing-templates.md

@@ -533,6 +533,11 @@ catalogFilter:
     metadata.annotations.github.com/team-slug: { exists: true }
 ```
 
+#### Custom validation messages
+
+You may specify custom JSON Schema validation messages as supported by the
+[ajv-errors](https://github.com/ajv-validator/ajv-errors) plugin library to [ajv](https://github.com/ajv-validator/ajv).
+
 ## `spec.steps` - `Action[]`
 
 The `steps` is an array of the things that you want to happen part of this