JSON output: better error reporting

[axic]

I think it would make sense changing the "error" field to a map of:

• severity (warning or error)
• message
• location
  • source file name (or pointer to the source object / identifier)
  • source line

The improvement would be twofold:

• No need to manually trying to assess the severity
• Proper link to the source file

[chriseth]

Hm, how is that different from the way it is currently done?

[axic]

#1387 provides this layout

[axic]

Closed by #1387.