Validates a gateway exception level and returns the value to render (or undefined to leave the
service default). A missing value (undefined/null from a TS or YAML/jsii caller) is accepted as
"unset". Any other value outside GatewayExceptionLevel is rejected at synth — naming the
value and the accepted set — rather than let the service roll the stack back at deploy.
Validates a gateway exception level and returns the value to render (or
undefinedto leave the service default). A missing value (undefined/null from a TS or YAML/jsii caller) is accepted as "unset". Any other value outside GatewayExceptionLevel is rejected at synth — naming the value and the accepted set — rather than let the service roll the stack back at deploy.