Fix TestCommandScript: return an error if target executable is not set
Fix TestCommandScript: return an error if target executable is not set

The test invokes the 'targetname' test command before setting a
target executable, which caused Python to raise TypeError: cannot
concatenate 'str' and 'NoneType' objects.

llvm.org/pr23686


git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@238425 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb