Make the add_custom_command output more predictable

I otherwise get:

   Expected stderr to match:

    expect-err> CMake Error at AppendNotOutput.cmake:1 \(add_custom_command\):
    expect-err>   add_custom_command given APPEND option with output.*
    expect-err>   which is not already a custom command output.
    expect-err> Call Stack \(most recent call first\):
    expect-err>   CMakeLists.txt:3 \(include\)

   Actual stderr:

    actual-err> CMake Error at AppendNotOutput.cmake:1 (add_custom_command):
    actual-err>   add_custom_command given APPEND option with output
    actual-err>   "/home/stephen/dev/src/cmake/with
    actual-err> space/Tests/RunCMake/add_custom_command/AppendNotOutput-build/out" which is
    actual-err>   not already a custom command output.
    actual-err> Call Stack (most recent call first):
    actual-err>   CMakeLists.txt:3 (include)

Using a specific line for paths is a style already used elsewhere for
the same reason, such as CMP0041 output.

Source/cmAddCustomCommandCommand.cxx

@@ -263,8 +263,8 @@ bool cmAddCustomCommandCommand::InitialPass(
 
     // No command for this output exists.
     std::ostringstream e;
-    e << "given APPEND option with output \"" << output[0]
-      << "\" which is not already a custom command output.";
+    e << "given APPEND option with output\n\"" << output[0]
+      << "\"\nwhich is not already a custom command output.";
     this->SetError(e.str());
     return false;
   }

Tests/RunCMake/add_custom_command/AppendNotOutput-stderr.txt

@@ -1,5 +1,8 @@
 CMake Error at AppendNotOutput.cmake:1 \(add_custom_command\):
-  add_custom_command given APPEND option with output.*
+  add_custom_command given APPEND option with output
+
+  .*RunCMake/add_custom_command/AppendNotOutput-build/out.*
+
   which is not already a custom command output.
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)