impl(bigquery): add query errors

src/bigquery/Cargo.toml

@@ -31,9 +31,14 @@ serde_json.workspace       = true
 google-cloud-auth          = { workspace = true }
 google-cloud-gax.workspace = true
 google-cloud-bigquery-v2   = { workspace = true }
-wkt                        = { workspace = true }
+thiserror.workspace        = true
+wkt.workspace              = true
 gaxi                       = { workspace = true, features = ["_internal-common", "_internal-grpc-client", "_internal-http-client"] }
 
+[dev-dependencies]
+anyhow.workspace = true
+tokio            = { workspace = true, features = ["macros", "rt-multi-thread"] }
+
 [features]
 default = ["default-rustls-provider"]
 # Enabled by default. Use the default rustls crypto provider ([aws-lc-rs]) for
@@ -42,9 +47,5 @@ default = ["default-rustls-provider"]
 # default and call `rustls::CryptoProvider::install_default()`.
 default-rustls-provider = ["gaxi/_default-rustls-provider"]
 
-[dev-dependencies]
-anyhow.workspace = true
-tokio            = { workspace = true, features = ["macros", "rt-multi-thread"] }
-
 [lints]
 workspace = true

src/bigquery/src/error.rs

@@ -0,0 +1,88 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Custom errors for the Cloud BigQuery query client.
+
+use google_cloud_bigquery_v2::model::ErrorProto;
+use google_cloud_gax::error::Error;
+
+/// Errors that can occur during query configuration, execution, or polling.
+#[derive(thiserror::Error, Debug)]
+#[non_exhaustive]
+pub enum QueryError {
+    /// The project ID was not provided or could not be determined.
+    #[error("no project ID was provided")]
+    MissingProjectId,
+
+    /// Only query jobs are supported by this client.
+    #[error("only query jobs are supported")]
+    UnsupportedJobType,
+
+    /// The query job failed on the BigQuery service side.
+    /// Includes the list of error protocols returned by the service.
+    #[error("query job failed: {reason} - {message}")]
+    JobFailed {
+        /// The primary error reason code (e.g., "invalidQuery", "backendError").
+        reason: String,
+        /// The error message.
+        message: String,
+        /// The list of all errors associated with the job.
+        errors: Vec<ErrorProto>,
+    },
+
+    /// The underlying RPC failed.
+    #[non_exhaustive]
+    #[error("the operation failed. RPC error: {source}")]
+    Rpc {
+        /// The error returned by the service for the request.
+        #[source]
+        source: Error,
+    },
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_job_failed_display() {
+        let err = QueryError::JobFailed {
+            reason: "invalidQuery".to_string(),
+            message: "Syntax error: Unexpected end of input".to_string(),
+            errors: vec![
+                ErrorProto::new()
+                    .set_reason("invalidQuery")
+                    .set_message("Syntax error: Unexpected end of input"),
+            ],
+        };
+        assert_eq!(
+            err.to_string(),
+            "query job failed: invalidQuery - Syntax error: Unexpected end of input"
+        );
+    }
+
+    #[test]
+    fn test_rpc_display() {
+        let status = google_cloud_gax::error::rpc::Status::default()
+            .set_code(google_cloud_gax::error::rpc::Code::InvalidArgument)
+            .set_message("simulated bad request");
+        let err = QueryError::Rpc {
+            source: Error::service(status),
+        };
+        assert_eq!(
+            err.to_string(),
+            "the operation failed. RPC error: the service reports an error with code INVALID_ARGUMENT described as: simulated bad request"
+        );
+    }
+}

src/bigquery/src/lib.rs

@@ -26,6 +26,7 @@
 
 pub use google_cloud_gax::Result;
 pub use google_cloud_gax::error::Error;
+pub mod error;
 
 pub(crate) use google_cloud_gax::client_builder::Result as ClientBuilderResult;