Define behavior when filters throw

Define the behavior when an exception_filter expression throws an exception.

The roslyn compiler generates a `filter` block for the exception filter. The runtime produces this behavior for a filter block.

Author: BillWagner

standard/statements.md

@@ -1749,7 +1749,7 @@ It is a compile-time error for a `return` statement to occur in a `finally` bloc
 
 When execution reaches a `try` statement, control is transferred to the `try` block. If control reaches the end point of the `try` block without an exception being propagated, control is transferred to the `finally` block if one exists. If no `finally` block exists, control is transferred to the end point of the `try` statement.
 
-If an exception has been propagated, the `catch` clauses, if any, are examined in lexical order seeking the first match for the exception. The search for a matching `catch` clause continues with all enclosing blocks as described in [§13.10.6](statements.md#13106-the-throw-statement). A `catch` clause is a match if the exception type matches any *exception_specifier* and any *exception_filter* is true. A `catch` clause without an *exception_specifier* matches any exception type. The exception type matches the *exception_specifier* when the *exception_specifier* specifies the exception type or a base type of the exception type. If the clause contains an exception filter, the exception object is assigned to the exception variable, and the exception filter is evaluated.
+If an exception has been propagated, the `catch` clauses, if any, are examined in lexical order seeking the first match for the exception. The search for a matching `catch` clause continues with all enclosing blocks as described in [§13.10.6](statements.md#13106-the-throw-statement). A `catch` clause is a match if the exception type matches any *exception_specifier* and any *exception_filter* is true. A `catch` clause without an *exception_specifier* matches any exception type. The exception type matches the *exception_specifier* when the *exception_specifier* specifies the exception type or a base type of the exception type. If the clause contains an exception filter, the exception object is assigned to the exception variable, and the exception filter is evaluated. If the evaluation of the *boolean_expression* for an *exception_filter* throws an exception, that exception is caught, and the exception filter evaluates to `false`.
 
 If an exception has been propagated and a matching `catch` clause is found, control is transferred to the first matching `catch` block. If control reaches the end point of the `catch` block without an exception being propagated, control is transferred to the `finally` block if one exists. If no `finally` block exists, control is transferred to the end point of the `try` statement. If an exception has been propagated from the `catch` block, control transfers to the `finally` block if one exists. The exception is propagated to the next enclosing `try` statement.