Improve the debug output a bit.

* src/debug.h (TRACE_ERR): Include the line number in the output.

doc/gpgme.texi

@@ -6089,7 +6089,8 @@ may use @acronym{GPGME}'s built in trace feature.  This feature is
 either enabled using the environment variable @code{GPGME_DEBUG} or,
 if this is not possible, by calling the function
 @code{gpgme_set_global_flag}.  The value is the trace level and
-an optional file name.
+an optional file name.  If no file name is given the trace output is
+printed to @code{stderr}.
 
 @noindent
 For example

src/debug.h

@@ -187,8 +187,8 @@ _gpgme_trace_gpgme_error (gpgme_error_t err, const char *file, int line)
 
 #define TRACE_ERR(err)							\
   err == 0 ? (TRACE_SUC ()) :						\
-    (_gpgme_debug (_gpgme_trace_level, "%s: error: %s <%s>\n",		\
-		   _gpgme_trace_func, gpgme_strerror (err),		\
+    (_gpgme_debug (_gpgme_trace_level, "%s:%d: error: %s <%s>\n",	\
+		   _gpgme_trace_func, __LINE__,  gpgme_strerror (err),  \
 		   gpgme_strsource (err)), _gpgme_debug_frame_end (), (err))
 /* The cast to void suppresses GCC warnings.  */
 #define TRACE_SYSRES(res)						\