Add error codes.

docs/topics/error_codes.md

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+# Error Codes
+
+Geometry Dash servers usually send an error code denoting an error either on the servers'
+or the client's side.
+
+## General Error Codes
+
+| code | name              |
+|------|-------------------|
+| `-1` | General (Generic) |
+| `-2` | No Result / Taken |
+
+## Specific Error Codes
+
+### [Register][register]
+
+| code  | name               | description                                       |
+|-------|--------------------|---------------------------------------------------|
+| `-2`  | Name Taken         | The name is taken.                                |
+| `-3`  | Email Used         | The email is already used.                        |
+| `-4`  | Invalid Name       | The name is invalid.                              |
+| `-5`  | Invalid Password   | The password is invalid.                          |
+| `-6`  | Invalid Email      | The email is invalid.                             |
+| `-7`  | Password Mismatch  | The passwords do not match.                       |
+| `-8`  | Password Too Short | The password is too short (minimum 6 characters). |
+| `-9`  | Name Too Short     | The name is too short (minimum 3 characters).     |
+| `-99` | Email Mismatch     | The emails do not match.                          |
+
+### [Login][login]
+
+| code  | name                        | description                                            |
+|-------|-----------------------------|--------------------------------------------------------|
+| `-8`  | Password Too Short          | The password is less than 6 characters long.           |
+| `-9`  | Name Too Short              | The name is less than 3 characters long.               |
+| `-10` | Linked to Different Account | Already logged into a different account.               |
+| `-11` | Login Failed                | Login credentials are incorrect.                       |
+| `-12` | Account Disabled            | The account has been disabled.                         |
+| `-13` | Linked to Different Steam   | One account is allowed per one steam account (unused). |
+
+### [Save][save]
+
+| code | name                 | description                               |
+|------|----------------------|-------------------------------------------|
+| `-2` | Login Failed         | Login credentials are wrong.              |
+| `-4` | Request Too Large    | Request to the server is too large.       |
+| `-5` | Bad Login Info       | Invalid login information was provided.   |
+| `-6` | Something Went Wrong | Something went wrong on the servers' end. |
+
+### [Suggest Level][suggest_level]
+
+| code | name                | description                            |
+|------|---------------------|----------------------------------------|
+| `-2` | Missing Permissions | Missing permissions to suggest levels. |
+
+### [Comment Level][comment_level]
+
+| code                        | name                  | description                                   |
+|-----------------------------|-----------------------|-----------------------------------------------|
+| `-10`                       | Permanent Comment Ban | Permanently banned from commenting on levels. |
+| `temp_{timeout}_{reason}`\* | Temporary Comment Ban | Temporarily banned from commenting on levels. |
+
+\* `timeout` is the remaining duration of the ban in seconds, `reason` is the reason for getting banned.
+
+[register]: /endpoints/accounts/register
+[login]: /endpoints/accounts/login
+[save]: /endpoints/accounts/save
+[suggest_level]: /endpoints/levels/suggest_level
+[comment_level]: /endpoints/levels/comment_level