docs($parse): add missing error documents

Author: petebacondarwin

docs/content/error/$parse/esc.ngdoc

@@ -0,0 +1,10 @@
+@ngdoc error
+@name $parse:esc
+@fullName Value cannot be escaped
+@description
+
+Occurs when the parser tries to escape a value that is not known.
+
+This should never occur in practice. If it does then that indicates a programming
+error in the AngularJS `$parse` service itself and should be reported as an issue
+at https://github.com/angular/angular.js/issues.

docs/content/error/$parse/lval.ngdoc

@@ -0,0 +1,13 @@
+@ngdoc error
+@name $parse:lval
+@fullName Trying to assign a value to a non l-value
+@description
+
+Occurs when an expression is trying to assign a value to a non-assignable expression.
+
+This can happen if the left side of an assigment is not a valid reference to a variable
+or property. E.g. In the following snippet `1+2` is not assignable.
+
+```
+(1+2) = 'hello';
+```