modified 2 'new Error'-occurences to 'throw new Error' (so it becomes…

[coderofsalvation]

… catchable, like the other)

[flosse]

@coderofsalvation the patch seems to contain an invalid token :-
Would you mind to fix that?
And adding tests would be cool ;-)

[coderofsalvation]

ah yes, I wasn't really awake i guess.

SyntaxError: Store.es6.js: Unexpected token (163:16)
  161 |     let e, item;
  162 |     if (err) {
> 163 |       const e = throw new Error("could not load data");
      |                 ^
  164 |       if (typeof cb === "function") {
  165 |         return cb(e);
  166 |       } else {

actually Im not really sure what the intention is with using 'new Error' instead of just 'Error' and passing that to a callback.
The reason why i started this PR is because people who use this package indirectly might get confused with these error-messages.
These error messages come from json-file-store at the first run, when the file doesn't exist yet.
Do you have suggestions to prevent json-file-store dumping uncatchable Error-messages in the console?