Throw more descriptive error when user has not included pica in their html

[ShreyashKore]

Currently, if pica is not included in the HTML we get null is not subtype of Function which might not be clear to new users.

[CaiJingLong]

Good job.

In this case, should we provide clearer guidance in error messages?
For example, tell users where to view documents or how to add them?

[ShreyashKore]

I think guiding users to read documentation seems better than providing a link to pica.js. Do you want me to add link to pub.dev page or the github readme?