fix: Add missing error message when an error occurred (open-feature#256)

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## This PR
- Add missing error message when an error occurred

### Related Issues
Fixes open-feature#255

---------

Signed-off-by: Thomas Poignant <thomas.poignant@gofeatureflag.org>

src/OpenFeature/OpenFeatureClient.cs

@@ -262,7 +262,7 @@ private async Task<FlagEvaluationDetails<T>> EvaluateFlag<T>(
             {
                 this._logger.LogError(ex, "Error while evaluating flag {FlagKey}", flagKey);
                 var errorCode = ex is InvalidCastException ? ErrorType.TypeMismatch : ErrorType.General;
-                evaluation = new FlagEvaluationDetails<T>(flagKey, defaultValue, errorCode, Reason.Error, string.Empty);
+                evaluation = new FlagEvaluationDetails<T>(flagKey, defaultValue, errorCode, Reason.Error, string.Empty, ex.Message);
                 await this.TriggerErrorHooks(allHooksReversed, hookContext, ex, options).ConfigureAwait(false);
             }
             finally

test/OpenFeature.Tests/OpenFeatureClientTests.cs

@@ -178,6 +178,7 @@ public async Task OpenFeatureClient_Should_Return_DefaultValue_When_Type_Mismatc
 
             var evaluationDetails = await client.GetObjectDetails(flagName, defaultValue);
             evaluationDetails.ErrorType.Should().Be(ErrorType.TypeMismatch);
+            evaluationDetails.ErrorMessage.Should().Be(new InvalidCastException().Message);
 
             _ = mockedFeatureProvider.Received(1).ResolveStructureValue(flagName, defaultValue, Arg.Any<EvaluationContext>());