Fix crash when adding instrument with a float as aperture or focal_length

[WimDeMeester]

Fixes the crash that appears when adding an new instrument with aperture (or focal length) as a float instead of an integer.

@brickbots Can you add this to the next release?

[jscheidtmann]

I can still enter invalid numbers in Chrome, like "123.345e4443". The message displayed then says "Instrument added, restart PiFinder to use".

the error message in the log is:

2025-02-20 07:46:55,497 Webserver-Server:ERROR:Error adding instrument: invalid literal for int() with base 10: ''

[jscheidtmann]

Further note that on Safari (iPhone iOS 18.3.1) you can still enter other characters.
It's not crashing anymore.

If you change multiple entries and only one item is wrong, none of the changes is stored. Do we want that?

[WimDeMeester]

In the html input element, I have put number and 123.345e4443 is indeed a number, meaning 123.345 times 10 to the power 4443. Converting this to an integer will indeed not work, because an integer can not store such a large number. Maybe I can limit the aperture to some reasonable value, but the question is indeed what a reasonable value is...

[brickbots]

Thank you for helping to make this more robust!