add way to obtain error message in ILinkedLibManager, use it to display an error code for init lib checks

src/BizHawk.Client.EmuHawk/Program.cs

@@ -38,7 +38,7 @@ namespace BizHawk.Client.EmuHawk
 				var p = OSTC.LinkedLibManager.LoadOrZero(dllToLoad);
 				if (p == IntPtr.Zero)
 				{
-					using (var box = new ExceptionBox($"EmuHawk needs {desc} in order to run! See the readme on GitHub for more info. (EmuHawk will now close.)")) box.ShowDialog();
+					using (var box = new ExceptionBox($"EmuHawk needs {desc} in order to run! See the readme on GitHub for more info. (EmuHawk will now close.) Internal error message: {OSTC.LinkedLibManager.GetErrorMessage()}")) box.ShowDialog();
 					Process.GetCurrentProcess().Kill();
 					return;
 				}

src/BizHawk.Common/OSTailoredCode.cs

@@ -91,6 +91,8 @@ namespace BizHawk.Common
 
 			/// <exception cref="InvalidOperationException">could not find library</exception>
 			IntPtr LoadOrThrow(string dllToLoad);
+
+			string GetErrorMessage();
 		}
 
 		private class UnixMonoLLManager : ILinkedLibManager
@@ -128,6 +130,12 @@ namespace BizHawk.Common
 				return ret != IntPtr.Zero ? ret : throw new InvalidOperationException($"got null pointer from {nameof(dlopen)}, error: {Marshal.PtrToStringAnsi(dlerror())}");
 			}
 
+			public string GetErrorMessage()
+			{
+				var errCharPtr = dlerror();
+				return errCharPtr == IntPtr.Zero ? "No error present" : Marshal.PtrToStringAnsi(errCharPtr);
+			}
+
 			private const int RTLD_NOW = 2;
 		}
 
@@ -163,7 +171,9 @@ namespace BizHawk.Common
 			{
 				var ret = LoadOrZero(dllToLoad);
 				return ret != IntPtr.Zero ? ret : throw new InvalidOperationException($"got null pointer from {nameof(LoadLibrary)}, error code: {GetLastError()}");
-			}
+			}
+
+			public string GetErrorMessage() => $"Error Code 0x{GetLastError():X8}";
 		}
 
 		public enum DistinctOS : byte