Improve type mismatch and unresolved variable exception messages

[hesara]

This change is 

[CLAassistant]

All committers have signed the CLA.

[denis-anisimov]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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src/main/java/com/vaadin/sass/internal/parser/LexicalUnitImpl.java, line 408 at r1 (raw file):

        if (getLexicalUnitType() == SCSS_VARIABLE) {
            // still unresolved
            throw new UnresolvedReferenceException("unresolved:" + this);

Is this message is informative enough ?
What's this.toString() here? Is it enough to understand its nature ? Is it variable or what ?

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src/main/java/com/vaadin/sass/internal/parser/LexicalUnitImpl.java, line 412 at r1 (raw file):

        if (another.getLexicalUnitType() == SCSS_VARIABLE) {
            // still unresolved
            throw new UnresolvedReferenceException("unresolved:" + another);

Same here.

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Comments from Reviewable