Normalize log response

[tortillaj]

The GET stream method sometimes returns JSON and sometimes returns plain text. This makes it more tedious to handle responses.

If a log is not found, JSON is returned where the error property is a string: https://github.com/ProboCI/loom/blob/master/lib/api/controllers/index.js#L136

If a log is found, the plain text log is sent back:
https://github.com/ProboCI/loom/blob/master/lib/api/controllers/index.js#L153

If there is an exception or other error, JSON is returned where the error property is limited to only the exception error message:
https://github.com/ProboCI/loom/blob/master/lib/api/controllers/index.js#L19

Proposal - All responses return JSON, leaving properties empty that do not exist for the given response:

{
  error: exception or null, 
  errorMessage: error text (placed here separately in case there is no exception object), 
  log: log text or empty string
}