Error should be raised when we try to set slider of output variable

[danieljaeim]

Currently the application has functionality to set sliders during burst capture, but we are currently unwittingly able to set the slider for our output variable.

Take the function y = mx + b (great function, very chique)

Once we try burst capturing, if we accidentally set the slider index to be 1, which refers to the expression itself, we end up erasing the expression by simply setting the value of the expression to be the slider value.

It would be nice if we could raise an error message after detecting whether our slider index is set to an expression, rather than a slider. This might require further information about how the calculator parses expressions, but otherwise we should be able to set an error if we try manipulating a slider value onto an expression!

[japamat0]

@alissarenz @danieljaeim Does it make sense to to provide the user with a list of valid variables which can be adjusted for gif output instead of validating the index?