Crash if a config file isn't there.

If a configuration file is specified via the command line but does not exist,
fail loudly and crash to preserve the current state. Do not fail if the requested
file is the default configuration file, _config.yml. In that case, fall back on
the defaults.

lib/jekyll/configuration.rb

@@ -102,7 +102,10 @@ module Jekyll
     def config_files(override)
       # Get configuration from <source>/_config.yml or <source>/<config_file>
       config_files = override.delete('config')
-      config_files = File.join(source(override), "_config.yml") if config_files.to_s.empty?
+      if config_files.to_s.empty?
+        config_files = File.join(source(override), "_config.yml")
+        @default_config_file = true
+      end
       config_files = [config_files] unless config_files.is_a? Array
       config_files
     end
@@ -117,6 +120,14 @@ module Jekyll
       raise "Configuration file: (INVALID) #{file}".yellow if !next_config.is_a?(Hash)
       Jekyll.logger.info "Configuration file:", file
       next_config
+    rescue SystemCallError
+      if @default_config_file
+        Jekyll::Logger.warn "Configuration file:", "none"
+        {}
+      else
+        Jekyll::Logger.error "Fatal:", "The configuration file '#{file}' could not be found."
+        exit(1)
+      end
     end
 
     # Public: Read in a list of configuration files and merge with this hash
@@ -133,9 +144,6 @@ module Jekyll
           new_config = read_config_file(config_file)
           configuration = configuration.deep_merge(new_config)
         end
-      rescue SystemCallError
-        # Errno:ENOENT = file not found
-        Jekyll.logger.warn "Configuration file:", "none"
       rescue => err
         Jekyll.logger.warn "WARNING:", "Error reading configuration. " +
                      "Using defaults (and options)."