Using a CDN isn't the problem on it's own; the problem is the (unfortunately common) shoddy programming practice of assuming error never happen. Good programming almost always involves checking for and properly handling errors. Using the returned value from fopen(3) without checking if it was NULL probably results in the program crashing when displaying a simple "file not found" error message would have been more appropriate.
This principle is also true when writing web pages; if you load an external resource, you need to check for and handle the case where that resource might not be available. Failing to load a resource can happen for many different reasons, not just someone blocking it with their client. Proper handling the error depends on what the resource is: display the content that is already available (perhaps in a less than ideal state), or if that isn't possible displaying an appropriate error message. This includes cases where the Javascript itself isn't available (for any reason).
Networks are not reliable or universally available. Quality programming understand that and does the best it can when failures happen.
This principle is also true when writing web pages; if you load an external resource, you need to check for and handle the case where that resource might not be available. Failing to load a resource can happen for many different reasons, not just someone blocking it with their client. Proper handling the error depends on what the resource is: display the content that is already available (perhaps in a less than ideal state), or if that isn't possible displaying an appropriate error message. This includes cases where the Javascript itself isn't available (for any reason).
Networks are not reliable or universally available. Quality programming understand that and does the best it can when failures happen.