fix coin install bugs

lib/blockchain/common.js

@@ -63,7 +63,7 @@ function writeSupervisorConfig (coinRec, cmd) {
   const blockchain = coinRec.code
 
   const supervisorConfig = `[program:${blockchain}]
-command=${cmd}
+command=nice ${cmd}
 autostart=true
 autorestart=true
 stderr_logfile=/var/log/supervisor/${blockchain}.err.log
@@ -99,6 +99,7 @@ function writeFile (path, content) {
   } catch (err) {
     if (err.code === 'EEXIST') {
       logger.info(`${path} exists, skipping.`)
+      return
     }
 
     throw err