Improve error message

[martinmeow]

When validationMetadatasToSchemas fails to generate the spec the thrown error has very little information.

The class causing the error looks like this:

export class AssessmentItemDto {
  @IsString()
  @MaxLength(100)
  key: string;

  @IsString()
  @MaxLength(50)
  label: string;

  @IsOptional()
  @IsIn([true, false, null])
  value: boolean | null;

  @IsString()
  @MaxLength(100)
  remark: string;
}

The error is caused by @IsIn([true, false, null]). The error message is TypeError: undefined is not an object (evaluating 'item.type'). It would be nice if is said something like this:

Failed to generate spec while evaluating:
  class: `AssessmentItemDto`,
  property: `value`,
  decorator:`@IsIn`,
  message: `TypeError: undefined is not an object (evaluating 'item.type')`
  at: (file name and line number)