Add error details to raised validation errors

[vfonic]

WebFlow API sometimes provides the problems array field which sometimes adds more details about the returned error.
In my case, I was receiving "ValidationError" back, but I didn't know which field failed validation and why.

This PR adds the problems array to the raised error message so that the users of webflow-ruby can get more information into what caused the error.

https://developers.webflow.com/#errors

PS This was difficult to test. I ran the test by connecting to my WebFlow site and then manually changed the VCR cassette. Perhaps it would be best to re-record this after you add the validation on name to be on a single line.

Before

Webflow::Error: Validation Failure
/Users/viktor/.rvm/gems/ruby-2.6.6/gems/webflow-ruby-1.2.0/lib/webflow/client.rb:135:in `request'
/Users/viktor/.rvm/gems/ruby-2.6.6/gems/webflow-ruby-1.2.0/lib/webflow/client.rb:113:in `post'
/Users/viktor/.rvm/gems/ruby-2.6.6/gems/webflow-ruby-1.2.0/lib/webflow/client.rb:94:in `create_item'

After

Webflow::Error: Validation Failure
Field 'name': Expected value to be a single line: 'Monday:\r\n' +
  'Fever\r\n'
/Users/viktor/Developer/Ruby/webflow-ruby/lib/webflow/client.rb:135:in `request'
/Users/viktor/Developer/Ruby/webflow-ruby/lib/webflow/client.rb:113:in `post'
/Users/viktor/Developer/Ruby/webflow-ruby/lib/webflow/client.rb:94:in `create_item'

[phoet]

did you see that the tests include webflow api credentials that use a test-page? https://github.com/penseo/webflow-ruby/blob/master/test/webflow_test.rb#L3-L6

you can use that to regenerate whatever is necessary (aka batteries included)

[vfonic]

@phoet yes, I can use that to access the API. However, WebFlow API doesn't allow you to create collections or fields in collections. I need a field that is set to be single line and then it fails validation with the validation message shown in problems field of returned JSON.

What I did was: I created such field in one of my sites and then ran the test, recorded that and changed couple of things like collection ID. I think perhaps the best way to make sure this is all well done, without any mistakes on my end, you could create a field and set it to be single line and then remove the VCR cassette recording and run the test to generate it yourself.

EDIT: I didn't do this test on name field. Now thinking of it, I don't think that name field type can be modified to have the validation of being a single line.