addded error codes and improved error handling

.github/docs/error/README.md

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+# Errors
+
+Here you will find all Error codes and its meaning. Error or status codes that are starting with an s are success codes.
+
+## Error codes
+
+### `eu006`
+
+**Meaning:** There is an error while changing the password. _HTTP-Code: **406**_
+
+**Solution:** Make sure that you have entered the correct previous password and that the double check passwords are matching.
+
+### `eu004`
+
+**Meaning:** There is an error while updating the settings. _HTTP-Code: **500**_
+
+**Solution:** This error should not occur in the frontend. If so, please create an issue in this repository.
+
+### `eu005`
+
+**Meaning:** There is an error while fetching the app settings. _HTTP-Code: **500**_
+
+**Solution:** This error should not occur in the frontend. If so, please create an issue in this repository.
+
+### `eu001`
+
+**Meaning:** Username or password is wrong. _HTTP-Code: **404**_
+
+**Solution:** Check the username and password and try again.
+
+### `eu002`
+
+**Meaning:** The user is deactivated. _HTTP-Code: **403**_
+
+**Solution:** Contact an admin to reactivate the account.
+
+### `eu003`
+
+**Meaning:** There is an error while updating the last login timestamp. _HTTP-Code: **500**_
+
+**Solution:** This error should not occur in the frontend. If so, please create an issue in this repository.
+
+### `ep001`
+
+**Meaning:** There is an error while creating a product. _HTTP-Code: **406**_
+
+**Solution:** Verify that all required fields are provided and valid, then try again.
+
+### `ep002`
+
+**Meaning:** There is an error while fetching products. _HTTP-Code: **406**_
+
+**Solution:** Try again later. If the error persists, create an issue in this repository.
+
+### `ep003`
+
+**Meaning:** There is an error while fetching a product. _HTTP-Code: **406**_
+
+**Solution:** Ensure the product exists and try again.
+
+### `ep004`
+
+**Meaning:** There is an error while updating the product amount. _HTTP-Code: **406**_
+
+**Solution:** Check the amount value and try again.
+
+### `ep005`
+
+**Meaning:** There is an error while updating a product. _HTTP-Code: **406**_
+
+**Solution:** Verify the product data and try again.
+
+### `ep006`
+
+**Meaning:** There is an error while deleting products. _HTTP-Code: **500**_
+
+**Solution:** Try again later. If the error persists, create an issue in this repository.
+
+### `es001`
+
+**Meaning:** There is an error while fetching storage locations. _HTTP-Code: **500**_
+
+**Solution:** Try again later. If the error persists, create an issue in this repository.
+
+### `es000`
+
+**Meaning:** The request body is invalid. _HTTP-Code: **400**_
+
+**Solution:** Provide a storage name. The description is optional.
+
+### `es002`
+
+**Meaning:** There is an error while creating a storage location. _HTTP-Code: **500**_
+
+**Solution:** Try again later. If the error persists, create an issue in this repository.
+
+### `es003`
+
+**Meaning:** There is an error while updating a storage location. _HTTP-Code: **500**_
+
+**Solution:** Try again later. If the error persists, create an issue in this repository.
+
+### `es004`
+
+**Meaning:** There is an error while deleting a storage location. _HTTP-Code: **500**_
+
+**Solution:** Try again later. If the error persists, create an issue in this repository.