ldgen: catch exception with python3 compatible style

Closes https://github.com/espressif/esp-idf/issues/2720

tools/ldgen/ldgen.py

@@ -91,8 +91,7 @@ def main():
         script_model.fill(mapping_rules, sdkconfig)
 
         script_model.write(output_file)
-
-    except Exception, e:
+    except Exception as e:
         print("linker script generation failed for %s\nERROR: %s" % (input_file.name, e.message))
         # Delete the file so the entire build will fail; and not use an outdated script.
         os.remove(output_file.name)