Clarified error types

Tracked jmespath/jmespath.site/pull/116.

GRAMMAR.ABNF

@@ -33,6 +33,19 @@
 ;; Implementations can map the corresponding JSON types to their language equivalent. For example, a JSON null could map to
 ;; None in python, and nil in ruby and go.
 ;;
+;; # Errors
+;;
+;; Errors may be raised during the JMEspath evaluation process. How and when errors are raised is implementation specific,
+;; but implementations should indicate to the caller when errors have occurred.
+;;
+;; The following errors are defined:
+;;
+;; - `invalid-arity` is raised when an invalid number of function arguments is encountered during the evaluation process.
+;; - `invalid-type` is raised when an invalid type is encountered during the evaluation process.
+;; - `invalid-value` is raised when an invalid value is encountered during the evaluation process.
+;; - `not-a-number` is raised when arithmetic expressions overflow.
+;; - `unknown-function` is raised when an unknown function is encountered during the evaluation process.
+;;
 ;; # Grammar
 ;;
 ;; JMESPath grammar is specified using ABNF, as described in RFC4234