add C003 error description

Signed-off-by: simonsan <[email protected]>

src/SUMMARY.md

@@ -8,6 +8,7 @@
 - [Error Codes](./errors/intro.md)
   - [C001 Data corruption](./errors/C001.md)
   - [C002 Incorrect Password](./errors/C002.md)
+  - [C003 Verification failed](./errors/C003.md)
 - [Installation](./installation.md)
   - [Nightly Builds](./nightly_builds.md)
 - [Completions](./commands/misc/completions.md)

src/errors/C003.md

@@ -0,0 +1,18 @@
+# C003
+
+Verification failed: After decrypting and decompressing the data changed! The
+data may be corrupted.\nPlease check the backend for corruption and try again.
+You can also try to run `rustic check --read-data` to check for corruption. This
+may take a long time.
+
+## Possible Causes
+
+- The data has been corrupted.
+- The backend has a bug.
+
+## Solutions
+
+- Check the backend for corruption.
+- Try running `rustic check --read-data` to check for corruption.
+- If the error persists, please report the issue to the (backend) developers for
+  further investigation.