Check the error after Merge() and return if not nil

[sunilmhta]

Fixes

#2520

Changes

Added a check for err != nil after Merge call.

Need to be mentioned in CHANGELOG.md?

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• blobber:
• gosdk:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

Merging #2551 (160e0bc) into staging (b74d8a3) will increase coverage by 0.01%.
The diff coverage is 0.00%.

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@@             Coverage Diff             @@
##           staging    #2551      +/-   ##
===========================================
+ Coverage    28.78%   28.80%   +0.01%     
===========================================
  Files          380      380              
  Lines        63995    63996       +1     
===========================================
+ Hits         18419    18431      +12     
+ Misses       43403    43386      -17     
- Partials      2173     2179       +6     

Flag	Coverage Δ
Unit-Tests	28.80% <0.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
code/go/0chain.net/sharder/transaction.go	20.12% <0.00%> (-1.36%)	⬇️

... and 3 files with indirect coverage changes

[dabasov]

lgtm