Add error dialog for invalid function entry

[djmgit]

Presently when user enters an invalid function, an error is shown in the terminal. Please display a dialog box showing the error when the user enters an invalid function and tries to plot it.

[aditodkar]

I would like to work on this issue can you assign it to me. I am familiar with python and tkinter.

[djmgit]

@aditodkar Github will not allow me to assign you unless you have sent atleast one PR to this repo, don not worry, this issue has been claimed by you, please start working, good luck :)

[akshatnitd]

I would like to work on it

[djmgit]

@akshatnitd please go ahead with it as this issue is opened for a long time :)