I've written an enhanced version of the ERRORMSG.DLL that can be found in fasm examples. I had a less enhanced version for about a year, that helps me to catch bugs, but now I've added much more functionality. Since I've finished it just now, some bugs may still be there. Please, let me know about them and, in general, do you think it is useful? Is it clear how to use it, or a tutorial is needed?=)

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Agreed. We need more examples anyway IMO and this fits the description.

Sasha: If you wish to add a tutorial then please feel free to do that.

This is the first steps for creating a more powerful error handler, with logging to files and displaying the error log at program termination or by calling a specific function (Like ShowErrorLog) while still running. There should be a console version prior to logfile version, also, but I can't force it to work.
The most typical usage of this library:
1. Show a messagebox with "Error" title and "Undefined error" text:

2. Show messagebox with your own caption and message

3. Retrieve the last error code and translate it to error text message

This is enough to handle the most cases, but I've made more. SetLastErrorMessage works similar to the windows SetLastError, but it also sets the error message string. This can be done by creating a message tables (see http://msdn.microsoft.com/en-us/library/windows/desktop/ms680357(v=vs.85).aspx), but my way is much simpler. And it is one of the reasons why this will not work in multithreaded environment.

4. This can be called somewhere inside the procedure

and then after it returns:

5. SetLastErrorCaption function works the same, but you provide just the string, no error codes

and then

There are also miscellaneous functions that may be helpful in some situations.

In general: NULL means display the predefined message. 1 means display a message that was set by the call to the one of the SetLast... functions or provide your own string.

I thought a little and decided to remove all unnecessary things and to make the library really simple, but still more functional, than the original fasm example. So here it is ERRORMSG.DLL version 2.0
It seems to be multithread compatible.
This version has a different, much simplier usage and it is more close to the original dll. We have here 2 functions:
BOOL ShowLastError(HWND hWnd,LPCTSTR lpCaption,LPCTSTR lpErrorMessage);
BOOL ShowError(HWND hWnd,LPCTSTR lpCaption,DWORD dwErrCode);
Examples:
1. MessageBox with "Error" caption and retrieved last error message

2. No comments

3. If you set the last error code with 29 bit set you will get an "Undefined error" message. Until you specify the message table.

4. You have an error code, and just want to format it to sring? Use ShowError