exewrapper: report name of failed DLL in error message

This uses C string literal concatenation.

Example output:

  $ hg version
  abort: failed to load Python DLL python27.dll

Note that HGPYTHONLIB does not contain the trailing .dll, which works here
because the documentation of LoadLibrary [1] says, that if no file name
extension is specified in the filename parameter, the default library
extension .dll is appended.

See [2] for a motivation of this change.

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx
[2] https://selenic.com/pipermail/mercurial/2015-August/048627.html

mercurial/exewrapper.c

@@ -106,7 +106,7 b' int main(int argc, char *argv[])'
 	if (pydll == NULL) {
 		pydll = LoadLibrary(HGPYTHONLIB);
 		if (pydll == NULL) {
-			err = "failed to load Python DLL";
+			err = "failed to load Python DLL " HGPYTHONLIB ".dll";
 			goto bail;
 		}
 	}