[MC-279951] All successful /data commands modifying pre-existing storage files cause an "unexpected error" (and cancel any currently running functions)

Any data command that successfully modifies a storage target under any namespace that existed at world load (i.e. 'data/command_storage_<namespace>.dat' was present) will fail with “An unexpected error occurred trying to execute that command” (mouseover: “java.lang.UnsupportedOperationException”). If the actual storage target ID (full namespace:id pair) existed when the world was loaded, the modification will still take place like normal, otherwise the storage target will not be created and the change will be lost. (Edit: data merge seems to have no effect either way, even if data modify ... merge does.) If this happens inside of a function, the entire function stack stops executing.

Steps to reproduce:

data modify storage test:a a set value 1

Command runs successfully.

Close and reopen the world.
```
data modify storage test:a b set value 1
```
- “An unexpected error occurred trying to execute that command”
```
data modify storage test:a b set value 1
```
- Command fails as expected since b was just set to 1.
```
data remove storage test:a b
```
- “An unexpected error occurred trying to execute that command”
```
data remove storage test:a b
```
- Command fails as expected since b was just removed. (The error message doesn't make much sense, but that's unrelated.)
```
data modify storage test:b a set value 1
```
- “Unexpected error” and the test:b storage is not created, so no modification takes place.
```
data modify storage test:b a set value 1
```
- “Unexpected error” again because test:b is still empty.
```
data modify storage test_2:a a set value 1
```
- Command succeeds as normal since the test_2 namespace did not exist at world load/was not loaded from a file.