Gracefully handle signature change on overcommit --run

We were dumping a stack trace for `overcommit --run` when the
configuration file signature had changed since. This isn't a great user
experience, so handle the error and print the error message properly.

Author: sds

lib/overcommit/cli.rb

@@ -27,6 +27,9 @@ def run
       when :run_all
         run_all
       end
+    rescue Overcommit::Exceptions::ConfigurationSignatureChanged => ex
+      puts ex
+      exit 78 # EX_CONFIG
     rescue Overcommit::Exceptions::HookContextLoadError => ex
       puts ex
       exit 64 # EX_USAGE

lib/overcommit/configuration_loader.rb

@@ -68,6 +68,8 @@ def load_file(file)
       end
 
       config
+    rescue Overcommit::Exceptions::ConfigurationSignatureChanged
+      raise
     rescue => error
       raise Overcommit::Exceptions::ConfigurationError,
             "Unable to load configuration from '#{file}': #{error}",