fix #607, sometimes uglifyjs will generate wrong code when our code like this.

try {
  doSomething();
} catch(e) {
  errorHandling();
}

after uglifyjs, sometimes the code may become

try {
  doSomething();
} catch(e) {
  e();
}

although use screwie8 option can resolve such issue, but to use another variable name other than 'e' is much safe.