I think you have to take into account the consequences of a move, and how difficult it is to find the right move or at least avoid the wrong one, as well as the skill of the player making it. A blunder is a mistake so awful you wouldn´t normally expect a player of that strength to make it. A complete beginner can´t blunder as you expect them to leave pieces hanging and so forth. What I could pass off as an error much stronger players would see as a blunder. A blunder is any easily avoided mistake.
Apart from blunders there are two classes of mistake, in chess you cannot make a winning move, if a winning move is available the position in front of you is already winning - a mistake is any move which changes the result (which is aways for the worse) so if you were winning and make a mistake then you are no longer winning and the result has changed to either a drawn position or a lost one.
Most chess mistakes are inaccuracies. They don´t change the result, in that sense they aren´t mistakes, but they don´t put as much pressure on your opponent and make it easy for them to avoid making a result changing mistake of their own. For example, in the Slav defence 3. Nc3 is (probably) inaccurate as black can play 3. ... dxc4 and then you can get: 4. e4 b5 5. a4 b4 and the knight has to move, whereas after 3. Nf3 there is nothing on c3 to be attacked. 3. Nc3 certainly doesn´t change the result (i.e. lose), but 3. Nf3 is probably more accurate.
What can happen in a game is that you make what most people would regard as a more serious mistake, because it is more easily avoided, to put yourself in a worse, but non-losing, position, and then make a less serious mistake (or rather a mistake that is less easy to avoid) that changes the result of the game to lost. Objectively the second mistake is the more serious as that is the one that changed the result, but subjectively the first one is more serious.
The only objective criterion for a mistake is whether it changes the result or not - how difficult your game is made is subjective. This makes it difficult to find objective criteria for classifying chess mistakes - and probably a waste of time - what you are trying to do in a chess game is make your opponent´s position sufficiently difficult for them that they are induced into making a result changing mistake - after they´ve moved you don´t sit there wondering if it was an error or an inaccuracy, you work out how to exploit the weaknesses in their position - which exist whether they made a mistake or not. This gives them a difficult game (hopefully) and creates the conditions for them to make further mistakes.