Update lrs_stream.go fix use of wrong err

[alingse]

the origin err has been checked in line 233.

it is nil now, and after check CheckInvalid() != nil,

here should use the check err

RELEASE NOTES:

• xds: Fix reported error string when LRS load reporting interval is invalid.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.02%. Comparing base (51d6a43) to head (c808c95).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
xds/internal/xdsclient/transport/lrs/lrs_stream.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8224      +/-   ##
==========================================
- Coverage   82.18%   82.02%   -0.16%     
==========================================
  Files         410      412       +2     
  Lines       40236    40491     +255     
==========================================
+ Hits        33066    33214     +148     
- Misses       5824     5900      +76     
- Partials     1346     1377      +31     

Files with missing lines	Coverage Δ
xds/internal/xdsclient/transport/lrs/lrs_stream.go	70.55% <66.66%> (ø)

... and 50 files with indirect coverage changes

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[arjan-bal]

LGTM, thanks! Adding a second reviewer.