add errors docs

docs/pages/docs/reference/client/errors.md

@@ -0,0 +1,40 @@
+# Errors
+
+## ErrNotFound
+
+`ErrNotFound` is returned when a query does not return any results. This error may be returned in `FindUnique`, `FindFirst`, but also when updating or deleting single records using `FindUnique().Update()` and `FindUnique().Delete()`.
+
+```go
+post, err := client.Post.FindFirst(
+  db.Post.Title.Equals("hi"),
+).Exec(ctx)
+if err != nil {
+  if errors.Is(err, db.ErrNotFound) {
+    panic("no record with title 'hi' found")
+  }
+  panic("error occurred: %s", err)
+}
+```
+
+## IsUniqueConstraintViolation
+
+A unique constraint violation happens when a query attempts to insert or update a record with a value that already exists in the database, or in other words, violates a unique constraint.
+
+```go
+user, err := db.User.CreateOne(...).Exec(cxt)
+if err != nil {
+  if info, err := db.IsErrUniqueConstraint(); err != nil {
+    // Fields exists for Postgres and SQLite
+    log.Printf("unique constraint on the fields: %s", info.Fields)
+
+    // you can also compare it with generated field names:
+    if info.Fields[0] == db.User.Name.Field() {
+      // do something
+			log.Printf("unique constraint on the `user.name` field")
+    }
+
+    // For MySQL and MongoDB, use the constraint key
+    log.Printf("unique constraint on the key: %s", info.Key)
+  }
+}
+```