check that source exists (fixes #191)

pollen/private/ts.rktd

@@ -1 +1 @@
-1545271092
+1545591698

pollen/render.rkt

@@ -84,6 +84,8 @@
                              source-path
                              maybe-output-path
                              maybe-template-path)
+  (unless (file-exists? source-path)
+    (raise-argument-error caller "existing source path" source-path))
   (define output-path (or maybe-output-path (->output-path source-path)))
   (unless output-path
     (raise-argument-error caller "valid output path" output-path))
@@ -124,6 +126,8 @@
 
 (define+provide/contract (render source-path [maybe-template-path #f] [maybe-output-path #f])
   ((complete-path?) ((or/c #f complete-path?) (or/c #f complete-path?)) . ->* . (or/c string? bytes?))
+  (unless (file-exists? source-path)
+    (raise-argument-error 'render "existing source path" source-path))
   (define output-path (or maybe-output-path (->output-path source-path)))
   (unless output-path
     (raise-argument-error 'render "valid output path" output-path))