src: reduce redundant "error:" for `inspect` and `findrefs`

[oyyd]

When inspect or findrefs an invalid expr, llnode will print an error message prefixed with "error: error: error:". This PR will reduce one redundant "error:" for such scenario and it seems that we can't remove the other redundant "error:" by calling SB API only.

Before this PR:

(llnode) v8 inspect invalid_addr
error: error: error: use of undeclared identifier 'invalid_addr'
(llnode) v8 findrefs -v invalid_addr
error: error: error: use of undeclared identifier 'invalid_addr'

After:

(llnode) v8 inspect invalid_addr
error: error: use of undeclared identifier 'invalid_addr'
(llnode) v8 findrefs -v invalid_addr
error: error: use of undeclared identifier 'invalid_addr'

The other redundant "error:" seems to come from here and I couldn't find a way to remove it by calling SB API only.

[codecov-io]

Codecov Report

Merging #285 into master will increase coverage by 0.23%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #285      +/-   ##
=========================================
+ Coverage   80.56%   80.8%   +0.23%     
=========================================
  Files          33      33              
  Lines        4116    4120       +4     
=========================================
+ Hits         3316    3329      +13     
+ Misses        800     791       -9

Impacted Files	Coverage Δ
src/llnode.cc	75% <100%> (+1.87%)	:arrow_up:
src/llscan.cc	61.31% <100%> (+0.58%)	:arrow_up:
test/plugin/scan-test.js	98.97% <100%> (+0.06%)	:arrow_up:
src/llv8.cc	76.11% <0%> (-0.16%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 640799a...b678289. Read the comment docs.

[mmarchini]

Landed in 74414fc