Fix throwJSException in JSI for Node-API

[vmoroz]

Lately the stack overflow JSI test that runs JSI implementation for Node-API was failing.
The initial fix in PR #221 introduced a crash where in the isInstanceOf method we take a value from the vm::CallResult<bool> even if it has an error.
The crash became visible when started to use Clang compiler.
The more detailed analysis had shown that JSI for Node-API error reporting is significantly different from the base JSI implementation for Hermes. We must no use the isInstanceOf method at all.

In this PR we:

• implement the throwJSException so that it follows the JSI for Hermes implementation;
• fix the isInstanceOf method.

As a result the stack overflow test is passing.

[vmoroz]

/azp run

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