fix logic for identifying the subnet acl

[ratulm]

and warn instead of throwing a batfishexception

[batfish-bot]

This change is 

[dhalperi]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @dhalperi, @progwriter, and @ratulm)

---

projects/batfish/src/main/java/org/batfish/representation/aws/Subnet.java, line 170 at r1 (raw file):

    } else {
      warnings.redFlag("Could not find a network ACL for subnet " + _subnetId);
    }

what's the test this change fixes?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@8b6cacc). Click here to learn what that means.
The diff coverage is 70.58%.

@@            Coverage Diff            @@
##             master    #5139   +/-   ##
=========================================
  Coverage          ?   73.24%           
  Complexity        ?    31516           
=========================================
  Files             ?     2590           
  Lines             ?   127245           
  Branches          ?    15239           
=========================================
  Hits              ?    93196           
  Misses            ?    26742           
  Partials          ?     7307

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/batfish/representation/aws/Subnet.java	76.12% <68.96%> (ø)	30 <6> (?)
...ava/org/batfish/representation/aws/NetworkAcl.java	67.74% <80%> (ø)	27 <2> (?)

[dhalperi]

Reviewed 2 of 5 files at r2, 3 of 3 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @progwriter)