[#528] Don't crash when callstack points to another package

Description

Problem: When a cleveland test fails, and the callstack points to a module in a different package (i.e., a package that is not the test suite currently being run), cleveland will try to read the source code from that module and fail.

Solution: if the file cannot be open (e.g. because it's part of a different package), fail gracefully and show only the original error message and callstack.

This command:

stack test baseDAO-ligo-tests --fast --file-watch --test-arguments "--pattern \"\$0 \~ /can drop proposals/\""

Used to fail with:

Main.hs
  BaseDAO propose/vote entrypoints tests:
    Admin:
      can drop proposals: FAIL
        Exception: src/BaseDAO/ShareTest/Proposal/Flush.hs: openFile: does not exist (No such file or directory)

Now it fails with:

  BaseDAO propose/vote entrypoints tests:
    Admin:
      can drop proposals: FAIL (0.33s)
        Unexpected interpreter error.
        Reason: Wrong type for custom error: string
        Got: Michelson interpreter failed for contract KT1KCxn8Bv38gUQtEYUCy1hxUUiKbquqsf8j (BaseDAO): Reached FAILWITH instruction with "FAIL_DROP_PROPOSAL_NOT_ACCEPTED"
        
        CallStack (from HasCallStack):
          expectCustomError_, called at src/BaseDAO/ShareTest/Proposal/Flush.hs:341:7 in baseDAO-0.1.0.0-6g5Skklv0pIL9LKn9HOR0:BaseDAO.ShareTest.Proposal.Flush

You can try running this command locally against the HEAD of the branch martoon/test-tree-extraction-problem in baseDAO

Related issue(s)

Resolves #528 (closed)

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