Fixed code so it throws the right type of exception.

dlib/image_saver/save_png.h

@@ -4,7 +4,7 @@
 #define DLIB_SAVE_PnG_H__
 
 #include "save_png_abstract.h"
-#include "../image_loader/image_loader.h"
+#include "image_saver.h"
 #include "dlib/array2d.h"
 #include "dlib/pixel.h"
 #include <cstdio>
@@ -62,7 +62,7 @@ namespace dlib
         /* Open the file */
         fp = fopen(file_name.c_str(), "wb");
         if (fp == NULL)
-            throw image_load_error("Unable to open " + file_name + " for writing.");
+            throw image_save_error("Unable to open " + file_name + " for writing.");
 
         /* Create and initialize the png_struct with the desired error handler
          * functions.  If you want to use the default stderr and longjump method,
@@ -75,7 +75,7 @@ namespace dlib
         if (png_ptr == NULL)
         {
             fclose(fp);
-            throw image_load_error("Error while writing PNG file " + file_name);
+            throw image_save_error("Error while writing PNG file " + file_name);
         }
 
         /* Allocate/initialize the image information data.  REQUIRED */
@@ -84,7 +84,7 @@ namespace dlib
         {
             fclose(fp);
             png_destroy_write_struct(&png_ptr,  NULL);
-            throw image_load_error("Error while writing PNG file " + file_name);
+            throw image_save_error("Error while writing PNG file " + file_name);
         }
 
         /* Set error handling.  REQUIRED if you aren't supplying your own
@@ -95,7 +95,7 @@ namespace dlib
             /* If we get here, we had a problem writing the file */
             fclose(fp);
             png_destroy_write_struct(&png_ptr, &info_ptr);
-            throw image_load_error("Error while writing PNG file " + file_name);
+            throw image_save_error("Error while writing PNG file " + file_name);
         }