no warnings for filenames that are too long

[JamesGKent]

i recently came up against an issue with the SPIFFS file system, mainly due to a lack of documentation, see PR: 2860 in which i had uploaded a file with a name longer than the filesystem allows, so SPIFFS.exists(filename) returned false even though the file was there.

i'm thinking would it be possible to update the upload tool so that a warning or error is raised when this occurs? i know this may not be trivial given that the limit can be changed in the header, but in this case a warning should be sufficient? at least then at upload time the user is given some kind of warning why it (may) not work?