Report notebook warnings (Qiskit#642)

The notebook testing script does not report any warnings raised in
notebooks. It's important to report these so we can fix deprecations
before they're removed from Qiskit, and to spot any unnecessary warnings
that might harm the user experience.

I've tried this in the past and the best method is to search the
executed notebook outputs for warnings. That's what I do here.

### Screenshots

<img width="698" alt="Screenshot 2024-01-18 at 12 45 19"
src="https://github.com/Qiskit/documentation/assets/36071638/39438185-5f70-4df3-b3e4-4f483481cd65">

scripts/nb-tester/test-notebook.py

@@ -12,6 +12,8 @@
 
 import argparse
 import sys
+import warnings
+import textwrap
 from dataclasses import dataclass
 from datetime import datetime
 from pathlib import Path
@@ -39,6 +41,49 @@ class ExecuteOptions:
     submit_jobs: bool
 
 
+@dataclass(frozen=True)
+class NotebookWarning:
+    cell_index: int
+    msg: str
+
+    def report(self):
+        """
+        Format warning and print it
+        """
+        message = f"Warning detected in cell {self.cell_index}:\n"
+        for line in self.msg.splitlines():
+            message += (
+                textwrap.fill(
+                    line, width=77, initial_indent=" │ ", subsequent_indent=" │ "
+                )
+                + "\n"
+            )
+        print_yellow(message, flush=True)
+
+
+def print_yellow(s: str, **kwargs):
+    """
+    Use ANSI escape codes to print yellow text
+    """
+    print(f"\033[0;33m{str}\033[0m", **kwargs)
+
+
+def extract_warnings(notebook: nbformat.NotebookNode) -> list[NotebookWarning]:
+    """
+    Detect warning messages in cell outputs
+    """
+    notebook_warnings = []
+    for cell_index, cell in enumerate(notebook.cells):
+        if not hasattr(cell, "outputs"):
+            continue
+        for output in cell.outputs:
+            if hasattr(output, "name") and output.name == "stderr":
+                notebook_warnings.append(
+                    NotebookWarning(cell_index=cell_index, msg=output.text)
+                )
+    return notebook_warnings
+
+
 def execute_notebook(path: Path, options: ExecuteOptions) -> bool:
     """
     Wrapper function for `_execute_notebook` to print status
@@ -49,16 +94,23 @@ def execute_notebook(path: Path, options: ExecuteOptions) -> bool:
         nbclient.exceptions.CellTimeoutError,
     )
     try:
-        _execute_notebook(path, options)
+        nb = _execute_notebook(path, options)
     except possible_exceptions as err:
         print("\r❌\n")
         print(err)
         return False
+
+    notebook_warnings = extract_warnings(nb)
+    if notebook_warnings:
+        print("\r⚠️")
+        [w.report() for w in notebook_warnings]
+        return False
+
     print("\r✅")
     return True
 
 
-def _execute_notebook(filepath: Path, options: ExecuteOptions) -> None:
+def _execute_notebook(filepath: Path, options: ExecuteOptions) -> nbformat.NotebookNode:
     """
     Use nbconvert to execute notebook
     """
@@ -73,12 +125,13 @@ def _execute_notebook(filepath: Path, options: ExecuteOptions) -> None:
     processor.preprocess(nb)
 
     if not options.write:
-        return
+        return nb
 
     for cell in nb.cells:
         # Remove execution metadata to avoid noisy diffs.
         cell.metadata.pop("execution", None)
     nbformat.write(nb, filepath)
+    return nb
 
 
 def find_notebooks(*, submit_jobs: bool = False) -> list[Path]: