fix: attach a catch handler to the handleContentP promise (#250)

This attaches a catch handler to the handleContentP promise, as in some
cases the flush is not called before node decides that this is an
unhandled promise rejection.

Fixes #249

lib/content/write.js

@@ -67,6 +67,7 @@ class CacacheWriteStream extends Flush {
         this.cache,
         this.opts
       )
+      this.handleContentP.catch(error => this.emit('error', error))
     }
     return this.inputStream.write(chunk, encoding, cb)
   }