Cannot Start MySQL in Yosemite

sudo vi /Library/LaunchDaemons/com.mysql.mysql.plist

produces the error:

ERROR! Manager of pid-file quit without updating file

Using Yosemite 10.10.5 migrated from another Mac where MySQL starts normally.

The com.mysql.mysql.plist file has the following content:
<?xml version="1.0" encoding="UTF-8"?>  
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> 
<plist version="1.0">  
  <dict>  
   <key>KeepAlive</key>  
   <true/>  
   <key>Label</key>  
   <string>com.mysql.mysqld</string>  
   <key>ProgramArguments</key>  
   <array>  
   <string>/usr/local/mysql/bin/mysqld_safe</string>  
   <string>--user=mysql</string>  
   </array>  
  </dict>  
</plist>

Terminal commands run successfully:
sudo chown root:wheel /Library/LaunchDaemons/com.mysql.mysql.plist
sudo chmod 644 /Library/LaunchDaemons/com.mysql.mysql.plist

Then:
sudo launchctl load -w /Library/LaunchDaemons/com.mysql.mysql.plist
returns the message:
/Library/LaunchDaemons/com.mysql.mysql.plist: Operation already in progress

Our terminal competence level is very basic. Could somebody please help?