[foreign] RFR: Fix UndefinedLayoutException exception message

[Jorn Vernee]

Hi,

JDK-8217245 changed the syntax for Unresolved layouts from `$(Name)` to 
`${Name}`. This also means Unresolved now has a dedicated 
`layoutExpression` field instead of relying on the name annotation.

LayoutResolver.UndefinedLayoutException is still using the name 
annotation in the exception message, which is now incorrect. This small 
patch fixes that, and changes it to use Unresolved::layoutExpression().

Please review.

Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/fixmessage/webrev.00/

Cheers,
Jorn