[NXCM-5498] Handle application/vnd.siesta-error-v1+json and application/vnd.siesta-validation-errors-v1+json nexus responses

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Handle application/vnd.siesta-validation-errors-v1+json response as an array of validation errors

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Add comments about parsing error responses

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #110 from sonatype/handle-siesta-errors-in-ui

REVIEW: [NXCM-5498] Handle application/vnd.siesta-error-v1+json and application/vnd.siesta-validation-errors-v1+json nexus responses