make setup.py stop with an error if config file cannot be found

This is the only sane behaviour, otherwise we don't know if our
build is misconfigured because 'build.conf' was silently used instead.

Fixes #133.

setup.py

@@ -45,10 +45,12 @@ def configure_extensions(exts, aliases=(), build_summary=None):
 
     #### Determine the name of the configuration file.
     config_file_option = '--configfile'
+    config_file_option_present = False
     # Handle command line option
     for i, opt in enumerate(sys.argv):
         if not opt.startswith(config_file_option):
             continue
+        config_file_option_present = True
         l, _, config_file = opt.partition('=')
         if l != config_file_option or not config_file:
             print('Error: Expecting {}=PATH'.format(config_file_option),
@@ -66,6 +68,10 @@ def configure_extensions(exts, aliases=(), build_summary=None):
             configs.read_file(f)
     except IOError:
         config_file_present = False
+        if config_file_option_present:
+            print("Error: '{}' option was provided, but '{}' does not exist"
+                  .format(config_file_option, config_file))
+            sys.exit(1)
     else:
         config_file_present = True