Raise error if ghostscript not installed

camelot/backends/ghostscript_backend.py

@@ -1,10 +1,40 @@
 # -*- coding: utf-8 -*-
 
+import sys
+import ctypes
+from ctypes.util import find_library
+
 import ghostscript
 
 
+def installed_posix():
+    library = find_library("gs")
+    return library is not None
+
+
+def installed_windows():
+    library = find_library(
+        "".join(("gsdll", str(ctypes.sizeof(ctypes.c_voidp) * 8), ".dll"))
+    )
+    return library is not None
+
+
 class GhostscriptBackend(object):
+    def installed(self):
+        if sys.platform in ["linux", "darwin"]:
+            return installed_posix()
+        elif sys.platform == "win32":
+            return installed_windows()
+        else:
+            return installed_posix()
+
     def convert(self, pdf_path, png_path, resolution=300):
+        if not self.installed():
+            raise OSError(
+                "Ghostscript is not installed. Please install it using the instructions"
+                "here: https://camelot-py.readthedocs.io/en/master/user/install-deps.html"
+            )
+
         gs_args = [
             "gs",
             "-q",