Print error message if the first cond pattern is ident pattern

[ericzhang6222]

Fixes #472 .

Changes proposed in this pull request:

• Print error message if the first cond pattern is ident pattern which will cause subsequent patterns unreachable

Checklist:

• [x] Added related tests
• [x] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build cee7c62ca9e8f0a585bf4e2bedf7934f3fba50dd-PR-479

• 0 of 7 (0.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.08%) to 47.595%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rel/expr_cond_pattern_control_var.go	0	7	0.0%
<!--	Total:	0	7	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
rel/value_set_generic.go	2	74.47%
rel/value_set_str.go	4	59.65%
<!--	Total:	6		-->

Totals
Change from base Build a54e1484615ecec9b3d6388de844c50bdc328ee4:	-0.08%
Covered Lines:	4295
Relevant Lines:	9024

---

💛 - Coveralls

[marcelocantos]

As a follow up, raise an issue to fail for all unreachable cases. E.g., the first condition renders the third condition unreachable in the following code:

cond {
    [x, ...]: x,
    (:a): a,
    [x, y]: x + y,
}

[ericzhang6222]

Close it, will rework and raise new PR in #484

[marcelocantos]

@ericzhang6222 do you still need the branch?

[ericzhang6222]

@ericzhang6222 do you still need the branch?

I have removed branch.