Added raise-support-error and exn:fail:support to racket/generic.

The exn:fail:support exception is used to signal "unsupported" values for
generic methods, e.g. a vector as argument to dict-remove.  Right interface,
just the wrong kind of instance.  The exception type helps define the notion of
a "supported" method, since a method might have a fallback implementation yet
some values are not considered "supported".

racket/collects/racket/generic.rkt

@@ -14,6 +14,8 @@
 
 (provide define-generics
          define/generic
+         raise-support-error
+         (struct-out exn:fail:support)
          chaperone-generics
          impersonate-generics
          redirect-generics

racket/collects/racket/private/dict.rkt

@@ -316,9 +316,6 @@
   (for/list ([k*v (in-dict-pairs d)])
     k*v))
 
-(define (raise-support-error name s)
-  (raise-mismatch-error name "not implemented for " s))
-
 (define-primitive-generics
   (dict gen:dict prop:gen:dict prop:gen:dict-methods dict? dict-implements?)
   #:fast-defaults

racket/collects/racket/private/generic.rkt

@@ -9,7 +9,9 @@
 
 (provide define-primitive-generics
          define-primitive-generics/derived
-         define/generic)
+         define/generic
+         raise-support-error
+         (struct-out exn:fail:support))
 
 (begin-for-syntax
 
@@ -383,11 +385,17 @@
        #'(define (method-name . method-formals)
            (define proc-name proc)
            (unless proc-name
-             (raise-arguments-error 'method-name
-                                    (format "not implemented for ~e"
-                                            self-name)))
+             (raise-support-error 'method-name self-name))
            method-apply))]))
 
+(struct exn:fail:support exn:fail [] #:transparent)
+
+(define (raise-support-error name v)
+  (raise
+   (exn:fail:support
+    (format "~a: not implemented for ~e" name v)
+    (current-continuation-marks))))
+
 (define-syntax (check-generic-method stx)
   (syntax-case stx ()
     [(check-generic-method

racket/collects/racket/private/set.rkt

@@ -386,9 +386,6 @@
        (set-remove! s x))]
     [else (raise-support-error 'set-symmetric-difference! s)]))
 
-(define (raise-support-error name s)
-  (raise-mismatch-error name "not implemented for " s))
-
 (define-sequence-syntax *in-set
   (lambda () #'in-set)
   (lambda (stx)