Remove all `panic` calls

[dcarbone]

There are a number of places in the codebase where I chose to raise a panic instead of error bubbling:

• values.go#L19
• values.go#L30
• attr.go#L21
• config.go#L121
• All conv typing functions.

This was very shortsighted on my part, and done before I had a good understanding of how to build a Terraform provider using the plugin framework. Ideally it would never panic, instead raising usable errors up to the end user.

This will require a bit of a rewrite of the util package and of providers which utilize it, thus will be reserved for v3.