tests/extmod/vfs_fat_fileio*: Improve skippability.

Should be skipped on missing uso, uerrno modules.

tests/extmod/vfs_fat_fileio1.py

 import sys
+try:
     import uerrno
     try:
         import uos_vfs as uos
         open = uos.vfs_open
     except ImportError:
         import uos
+except ImportError:
+    print("SKIP")
+    sys.exit()
+
 try:
     uos.VfsFat
 except AttributeError:

tests/extmod/vfs_fat_fileio2.py

 import sys
+try:
     import uerrno
     try:
         import uos_vfs as uos
         open = uos.vfs_open
     except ImportError:
         import uos
+except ImportError:
+    print("SKIP")
+    sys.exit()
+
 try:
     uos.VfsFat
 except AttributeError: