refactor: Log child process command output on error (serverless#679)

Author: zhemaituk

lib/pip.js

@@ -415,6 +415,13 @@ async function installRequirements(targetFolder, pluginInstance) {
             'PYTHON_REQUIREMENTS_COMMAND_NOT_FOUND'
           );
         }
+        if (log) {
+          log.info(`Stdout: ${e.stdoutBuffer}`);
+          log.info(`Stderr: ${e.stderrBuffer}`);
+        } else {
+          serverless.cli.log(`Stdout: ${e.stdoutBuffer}`);
+          serverless.cli.log(`Stderr: ${e.stderrBuffer}`);
+        }
         throw e;
       }
     }