Added 'Error' property with joined error messages (#82)

Co-authored-by: Alex Klaus <[email protected]>

README.md

@@ -136,6 +136,7 @@ It's built around `IDomainResult` interface that has 3 properties:
 
 ```csharp
 IReadOnlyCollection<string> Errors { get; } // Collection of error messages if any
+string Error { get; }                       // Error messages (if any) joined into a line for simplicity
 bool IsSuccess { get; }                     // Flag, whether the current status is successful or not
 DomainOperationStatus Status { get; }       // Current status of the domain operation: Success, Failed, NotFound, Unauthorized, etc.
 ```

src/Common/IDomainResultBase.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 
 namespace DomainResults.Common
 {
@@ -11,6 +12,12 @@ public interface IDomainResultBase
 		///		Collection of error messages if any
 		/// </summary>
 		IReadOnlyCollection<string> Errors { get; }
+
+		/// <summary>
+		///		Error messages joined into a single line ('.'-separated messages)
+		/// </summary>
+		string Error => !Errors.Any() ? string.Empty : string.Join(". ", Errors);
+
 		/// <summary>
 		///		Flag, whether the current status is successful or not
 		/// </summary>

tests/DomainResults.Tests/Common/IDomainResultTests.cs

@@ -17,7 +17,7 @@ public class IDomainResult_Tests
 
 	[Theory]
 	[MemberData(nameof(TestCasesWithNoValue))]
-	public void IDomainResult_Response_Test(Func<IDomainResult> method, DomainOperationStatus expectedStatus, IEnumerable<string> expectedErrMessages)
+	public void IDomainResult_Response_Test(Func<IDomainResult> method, DomainOperationStatus expectedStatus, string[] expectedErrMessages)
 	{
 		var domainResult = method();
 
@@ -26,6 +26,7 @@ public void IDomainResult_Response_Test(Func<IDomainResult> method, DomainOperat
 
 		Assert.Equal(expectedStatus, domainResult.Status);
 		Assert.Equal(expectedErrMessages, domainResult.Errors);
+		Assert.Equal(string.Join(". ", expectedErrMessages), domainResult.Error);
 	}
 
 	public static IEnumerable<object[]> TestCasesWithNoValue