Use attr_reader for read-only attributes

Minor cleanup to replace attr_accessor with attr_reader for read-only
attributes.

[#68122384]
Introduce surface error info from staging plugin

* Introduce staging_error key in the staging info file to capture staging
  error details
* Change plugin configuration to use a path for staging_info.yml instead
  of the base file name to avoid ambiguity
* Add exception classes that map to user-facing staging errors that can
  be surfaced through the cloud controller api
Surface staging error info in nats response to CC

Carry the detail about the source of staging problems to the cloud
controller.
Merge remote-tracking branch 'origin/pr/127'

[#71421004]
[#68122384]

Signed-off-by: Sabha Parameswaran <sparameswaran@gopivotal.com>