diff --git a/js-assembler/check-valid-module-source.rkt b/js-assembler/check-valid-module-source.rkt
index e8d7ad2..ac91eba 100644
--- a/js-assembler/check-valid-module-source.rkt
+++ b/js-assembler/check-valid-module-source.rkt
@@ -1,6 +1,7 @@
 #lang racket/base
 
-(provide check-valid-module-source)
+(provide check-valid-module-source
+         [struct-out exn:invalid-module-source])
 
 (require syntax/kerncase
          syntax/modresolve
@@ -9,9 +10,13 @@
          "../parser/path-rewriter.rkt")
 
 
-(define (abort-abort)
+(struct exn:invalid-module-source exn:fail ())
+
+
+(define (abort-abort #:reason (reason "Invalid module source"))
   (fprintf (current-report-port) "Aborting compilation.\n")
-  (exit))
+  (raise (exn:invalid-module-source reason
+                                    (current-continuation-marks))))
 
 
 (define ns (make-base-namespace))
diff --git a/whalesong-gui.rkt b/whalesong-gui.rkt
index fc9128f..f05acea 100644
--- a/whalesong-gui.rkt
+++ b/whalesong-gui.rkt
@@ -95,7 +95,7 @@
   (thread (lambda ()
             (parameterize ([current-report-port (open-output-text-editor t)])
               (build-html-and-javascript source-path)
-              (fprintf (current-report-port) "Build complete.")))))
+              (fprintf (current-report-port) "Done.\n")))))
 
 
 
diff --git a/whalesong-helpers.rkt b/whalesong-helpers.rkt
index 2c8d6ae..3f3ca9e 100644
--- a/whalesong-helpers.rkt
+++ b/whalesong-helpers.rkt
@@ -15,6 +15,7 @@
          "resource/structs.rkt"
          "logger.rkt"
          "parameters.rkt"
+         "js-assembler/check-valid-module-source.rkt"
          planet/version
          (for-syntax racket/base))
 
@@ -47,7 +48,13 @@
 
 (define (with-catchall-exception-handler thunk)
   (with-handlers
-      [(void (lambda (exn)
+      ([exn:invalid-module-source?
+        (lambda (exn)
+          (fprintf (current-report-port) "~a\n"
+                   (exn-message exn))
+          (fprintf (current-report-port) "------------------\n")
+          (fprintf (current-report-port) "\nAborting compilation.\n"))]
+       [void (lambda (exn)
                (fprintf (current-report-port) "ERROR: Whalesong has encountered an internal error.\n\n")
                (fprintf (current-report-port) "Please send the following error report log to dyoo@hashcollision.org.\n\n")
                (define op (open-output-string))
@@ -56,7 +63,7 @@
                (fprintf (current-report-port) "------------------\n")
                (displayln (get-output-string op) (current-report-port))
                (fprintf (current-report-port) "------------------\n")
-               (fprintf (current-report-port) "\nAborting compilation.\n")))]
+               (fprintf (current-report-port) "\nAborting compilation.\n"))])
     (thunk)))