Try harder to format error messages

Also marshals text as UTF8, as expected by libgit2.

Author: jamill

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -63,6 +63,7 @@
     <Compile Include="RefSpecFixture.cs" />
     <Compile Include="EqualityFixture.cs" />
     <Compile Include="RevertFixture.cs" />
+    <Compile Include="SetErrorFixture.cs" />
     <Compile Include="SignatureFixture.cs" />
     <Compile Include="FilterBranchFixture.cs" />
     <Compile Include="RemoveFixture.cs" />

LibGit2Sharp/Core/NativeMethods.cs

@@ -85,7 +85,7 @@ static NativeMethods()
         [DllImport(libgit2)]
         internal static extern void giterr_set_str(
             GitErrorCategory error_class,
-            string errorString);
+            [MarshalAs(UnmanagedType.CustomMarshaler, MarshalCookie = UniqueId.UniqueIdentifier, MarshalTypeRef = typeof(StrictUtf8Marshaler))] string errorString);
 
         [DllImport(libgit2)]
         internal static extern void giterr_set_oom();

LibGit2Sharp/Core/Proxy.cs

@@ -4,6 +4,7 @@
 using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
+using System.Text;
 using System.Threading;
 using LibGit2Sharp.Core.Handles;
 using LibGit2Sharp.Handlers;
@@ -23,7 +24,7 @@ public static void giterr_set_str(GitErrorCategory error_class, Exception except
             }
             else
             {
-                NativeMethods.giterr_set_str(error_class, exception.Message);
+                NativeMethods.giterr_set_str(error_class, ErrorMessageFromException(exception));
             }
         }
 
@@ -32,6 +33,63 @@ public static void giterr_set_str(GitErrorCategory error_class, String errorStri
             NativeMethods.giterr_set_str(error_class, errorString);
         }
 
+        /// <summary>
+        /// This method will take an exception and try to generate an error message
+        /// that captures the important messages of the error.
+        /// The formatting is a bit subjective.
+        /// </summary>
+        /// <param name="ex"></param>
+        /// <returns></returns>
+        public static string ErrorMessageFromException(Exception ex)
+        {
+            StringBuilder sb = new StringBuilder();
+            BuildErrorMessageFromException(sb, 0, ex);
+            return sb.ToString();
+        }
+
+        private static void BuildErrorMessageFromException(StringBuilder sb, int level, Exception ex)
+        {
+            string indent = new string(' ', level * 4);
+            sb.AppendFormat("{0}{1}", indent, ex.Message);
+
+            if (ex is AggregateException)
+            {
+                AggregateException aggregateException = ((AggregateException)ex).Flatten();
+
+                if (aggregateException.InnerExceptions.Count == 1)
+                {
+                    sb.AppendLine();
+                    sb.AppendLine();
+
+                    sb.AppendFormat("{0}Contained exception:{1}", indent, Environment.NewLine);
+                    BuildErrorMessageFromException(sb, level + 1, aggregateException.InnerException);
+                }
+                else
+                {
+                    sb.AppendLine();
+                    sb.AppendLine();
+
+                    sb.AppendFormat("{0}Contained exceptions:{1}", indent, Environment.NewLine);
+                    for (int i = 0; i < aggregateException.InnerExceptions.Count; i++)
+                    {
+                        if (i != 0)
+                        {
+                            sb.AppendLine();
+                        }
+
+                        BuildErrorMessageFromException(sb, level + 1, aggregateException.InnerExceptions[i]);
+                    }
+                }
+            }
+            else if (ex.InnerException != null)
+            {
+                sb.AppendLine();
+                sb.AppendLine();
+                sb.AppendFormat("{0}Inner exception:{1}", indent, Environment.NewLine);
+                BuildErrorMessageFromException(sb, level + 1, ex.InnerException);
+            }
+        }
+
         #endregion
 
         #region git_blame_