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(define-foreign-enum-type (exceptiontype int)
  (exceptiontype->int int->exceptiontype)
  ((UndefinedException) UndefinedException)
  ((WarningException) WarningException)
  ((ResourceLimitWarning) ResourceLimitWarning)
  ;; ... ellided
  ((ConfigureFatalError) ConfigureFatalError)
  ((PolicyFatalError) PolicyFatalError))

(define magick-get-exception
  (foreign-lambda c-string MagickGetException
                  (const magickwand)
                  (c-pointer exceptiontype)))

;; compiler output:

magickwand.c: In function ‘stub420’:
magickwand.c:122:1: warning: passing argument 2 of ‘MagickGetException’ from incompatible pointer type [enabled by default]
/usr/include/ImageMagick/wand/MagickWand.h:148:4: note: expected ‘enum ExceptionType *’ but argument is of type ‘int *’
  /usr/bin/csc -feature compiling-extension -setup-mode -k   -c -O3 -d1 -lMagickWand -C `pkg-config --cflags MagickWand` -L `pkg-config --
libs MagickWand` magickwand.scm -unit magickwand                                                                                         
magickwand.c: In function ‘stub420’:
magickwand.c:122:1: warning: passing argument 2 of ‘MagickGetException’ from incompatible pointer type [enabled by default]
/usr/include/ImageMagick/wand/MagickWand.h:148:4: note: expected ‘enum ExceptionType *’ but argument is of type ‘int *’
  /usr/bin/csc -feature compiling-extension -setup-mode -k   -s -O3 -d0 magickwand.import.scm -C `pkg-config --cflags MagickWand` -L `pkg-
config --libs MagickWand`                                                                                                                

Try changing
(define-foreign-enum-type (exceptiontype int) ...) to
(define-foreign-enum-type (exceptiontype (enum "ExceptionType")) ...)

what if you try 

(define magick-get-exception
  (foreign-lambda c-string MagickGetException
                  (const magickwand)
                  (c-pointer "ExceptionType")))