Sometimes corrupt files are left behind when "Save" As fails

[cmbruns]

This is a follow on to issue #10

Steps to reproduce:

1. Load the image from issue #10
2. "Save As"
3. Set output filter to "All Files"
4. Save file as a name without extension e.g. "phlerb"
5. An error dialog (correctly) appears
6. Notice the output file is created but not usable

Expected behavior:

• No output file is produced, because there was an error

[ckbruns]

Reproduced by saving file with .jpg extension - gives error, but also creates as unusable file Saving with no extension resulted in file saved as .png

[cmbruns]

@ckbruns please verify

[ckbruns]

Verified