This is mainly about error handling, simpler and more useful.

- Always warn when a valid SR can't be loaded, with a message that
  contains the original exception (for example, compared to before
  this commit, it will tell if a JSON file can't be found,
  or where JSON has syntax errors).

- Same as previous point if serialisation type is invalid or missing.
  If the caller needs it to transform parameters (parameters file),
  it will raise with the original exception. Otherwise, it falls back
  on json-in-xml. In some places, such fallback is a change of
  behaviour and I have no opinion about it except that at least
  it's now consistent throughout slapos.core.

- Remove warning about RootSoftwareInstance/default: meaningless
  because contradicted the comment, and useless because the transition
  to 'default' is already complete for SR schemas. There are still 3
  lines of backward compatibility code for the rest of slapos.core.

- Don't read the same file several times. Note however that this
  performance fix is only for the SoftwareReleaseSchema class:
  the caller should be fixed to not instanciate several times with
  the same parameters (from do_request & _requestComputerPartition).

See merge request nexedi/slapos.core!621