Improve error messages when creating an invalid PFFile.

[richardjrossiii]

Currently, if you attempt to create a PFFile with a nil NSData parameter, you get a strange 'file not found' error when attempting to save it to Parse.

Let's make this either

• Throw an exception
• Make the PFFile constructor return nil, when invalid data is passed in.

Somewhat relates to #6.

[nlutsenko]

Sounds great! Let's do it!
We have this method:

+ (instancetype)fileWithName:(PF_NULLABLE NSString *)name
                        data:(NSData *)data
                 contentType:(PF_NULLABLE NSString *)contentType
                       error:(NSError **)error;

Which returns nil and if there was an error - it will be populated.

What do you think if we take this approach:

• Check for nil on NSData (it's ok if it's empty probably, not ok if it's nil)
• Add more methods that accept pointer-to-a-pointer-to-an-error (nice name 😁)
• Mark methods without an error: parameter as unavailable in Swift, so whoever uses Swift gets throws :)

[nlutsenko]

This is probably an example of the method signature that we are aiming for in Swift:
public convenience init(name: String?, data: NSData, contentType: String?) throws
Which is a result of this in ObjC:

+ (PF_NULLABLE instancetype)fileWithName:(PF_NULLABLE NSString *)name
                                    data:(NSData *)data
                             contentType:(PF_NULLABLE NSString *)contentType PF_SWIFT_UNAVAILABLE;

+ (PF_NULLABLE instancetype)fileWithName:(PF_NULLABLE NSString *)name
                                    data:(NSData *)data
                             contentType:(PF_NULLABLE NSString *)contentType
                                   error:(NSError **)error;

[parse-github-assistant]

The label type:feature cannot be used in combination with type:improvement.