Improve vlog statements when PHYSFS_openRead fails

There are three different places where we call PHYSFS_openRead. This
commit makes sure all of them print a statement upon failure along with
the PhysFS reason for failure, and assigns the log level of each print
as so:

- FILESYSTEM_loadFileToMemory: Debug print (previously no
  print existed in the first place), because some files (such as
  font.txt) may or may not be needed, but if it isn't then no need to
  print and worry the user. The game will error anyway if a critical
  file like a graphics file is missing.

- FILESYSTEM_loadBinaryBlob: Debug print (previously info print),
  because sometimes it's not needed, such as mmmmmm.vvv. I remember one
  user being worried that the game printed "Unable to open file
  mmmmmm.vvv" when it's not critical unlike vvvvvvmusic.vvv (and that
  file is assumed to exist if data.zip exists anyways). Though maybe we
  should move to loose-leaf files to save on memory usage (and so we
  don't have to use special tools to modify a binary blob)...

- FILESYSTEM_loadZip: Error print. If we're calling this function, we
  really do expect the zip to be loaded, and if it fails because we
  can't open the file in the first place, then it would be good to know
  why.

desktop_version/src/FileSystemUtils.cpp

@@ -355,6 +355,14 @@ static bool FILESYSTEM_mountAssetsFrom(const char *fname)
 void FILESYSTEM_loadZip(const char* filename)
 {
     PHYSFS_File* zip = PHYSFS_openRead(filename);
+    if (zip == NULL)
+    {
+        vlog_error(
+            "Could not read zip %s: %s",
+            filename,
+            PHYSFS_getErrorByCode(PHYSFS_getLastErrorCode())
+        );
+    }
 
     if (!PHYSFS_mountHandle(zip, filename, "levels", 1))
     {
@@ -589,6 +597,11 @@ void FILESYSTEM_loadFileToMemory(
     handle = PHYSFS_openRead(name);
     if (handle == NULL)
     {
+        vlog_debug(
+            "Could not read file %s: %s",
+            name,
+            PHYSFS_getErrorByCode(PHYSFS_getLastErrorCode())
+        );
         goto fail;
     }
     length = PHYSFS_fileLength(handle);
@@ -673,7 +686,11 @@ bool FILESYSTEM_loadBinaryBlob(binaryBlob* blob, const char* filename)
     handle = PHYSFS_openRead(path);
     if (handle == NULL)
     {
-        vlog_info("Unable to open file %s", filename);
+        vlog_debug(
+            "Could not read binary blob %s: %s",
+            filename,
+            PHYSFS_getErrorByCode(PHYSFS_getLastErrorCode())
+        );
         return false;
     }