enhance(compiler): Python bindings: Forward llvm::Expected error messages in exceptions

The code in `lib/CAPI/Support/CompilerEngine.cpp` invokes several
functions returning an `llvm::Expected<T>`. When those fail, the error
message retrieved from the error object the `llvm::Expected<T>`
instance is written to the standard error stream via
`mlir::zamalang::log_error()` and an exception with a more generic
error message is thrown.

This causes errors to show up on the standard error stream in tests
generating errors on purpose and checking them, e.g.:

  ```
  tests/python/test_compiler_engine.py::test_compile_invalid[not
  @main] Compilation failed: cannot find the function for generate
  client parameters PASSED
  ```

This patch forwards the error message from an `llvm::Expected<T>`
instance in a runtime exception rather than writing it to the standard
error stream. Since exceptions are properly caught by the tests, no
errors show up during testing.

compiler/tests/python/test_compiler_engine.py

@@ -102,5 +102,5 @@ def test_compile_and_run_tlu(mlir_input, args, expected_result, tab_size):
 )
 def test_compile_invalid(mlir_input):
     engine = CompilerEngine()
-    with pytest.raises(RuntimeError, match=r"failed compiling"):
+    with pytest.raises(RuntimeError, match=r"Compilation failed:"):
         engine.compile_fhe(mlir_input)

compiler/tests/python/test_round_trip.py

@@ -69,5 +69,5 @@ def test_valid_mlir_inputs(mlir_input):
 @pytest.mark.parametrize("mlir_input", INVALID_INPUTS)
 def test_invalid_mlir_inputs(mlir_input):
     # We need to check that invalud inputs are raising an error
-    with pytest.raises(RuntimeError, match=r"mlir parsing failed"):
+    with pytest.raises(RuntimeError, match=r"MLIR parsing failed:"):
         compiler.round_trip(mlir_input)