Added error operator for custom validation #1259

pkg/yqlib/doc/operators/error.md

@@ -0,0 +1,56 @@
+# Error
+
+Use this operation to short-circuit expressions. Useful for validation.
+
+{% hint style="warning" %}
+Note that versions prior to 4.18 require the 'eval/e' command to be specified. 
+
+`yq e <exp> <file>`
+{% endhint %}
+
+## Validate a particular value
+Given a sample.yml file of:
+```yaml
+a: hello
+```
+then
+```bash
+yq 'select(.a == "howdy") or error(".a [" + .a + "] is not howdy!")' sample.yml
+```
+will output
+```bash
+Error: .a [hello] is not howdy!
+```
+
+## Validate the environment variable is a number - invalid
+Running
+```bash
+numberOfCats="please" yq --null-input 'env(numberOfCats) | select(tag == "!!int") or error("numberOfCats is not a number :(")'
+```
+will output
+```bash
+Error: numberOfCats is not a number :(
+```
+
+## Validate the environment variable is a number - valid
+`with` can be a convenient way of encapsulating validation.
+
+Given a sample.yml file of:
+```yaml
+name: Bob
+favouriteAnimal: cat
+```
+then
+```bash
+numberOfCats="3" yq '
+	with(env(numberOfCats); select(tag == "!!int") or error("numberOfCats is not a number :(")) | 
+	.numPets = env(numberOfCats)
+' sample.yml
+```
+will output
+```yaml
+name: Bob
+favouriteAnimal: cat
+numPets: 3
+```
+