Display absolute path for invalidBuildScriptPath and invalidTargetConfigFile error print. (#1353)

* Display absolute path in invalidBuildScriptPath error print.

With previous implementation, the error message was not clear for end user:
  Spec validation error: Target "Project" has a script "SwiftLint" which has a path that doesn't exist "../../wrong-dir/scripts/run_swiftlint_in_xcode_wrapper.sh"

With new implementation with absolute path, it is easier to validate what is wrong with path:
  Spec validation error: Target "Project" has a script "SwiftLint" which has a path that doesn't exist "/Users/SomeUser/dev/fancy-project/wrong-dir/scripts/run_swiftlint_in_xcode_wrapper.sh"

* Display absolute path for invalidTargetConfigFile error print.

Without patch:
  2 Spec validations errors:
	- Target "MM" has invalid config file "../../xcodegen/MM/Build/xcconfig/conan_config.xcconfig" for config "Debug"
	- Target "MM" has invalid config file "../../xcodegen/MM/Build/xcconfig/conan_config.xcconfig" for config "Release"

After fix:
  2 Spec validations errors:
	- Target "MM" has invalid config file path "/Users/MyUser/dev/myproject/xcodegen/MM/Build/xcconfig/conan_config.xcconfig" for config "Release"
	- Target "MM" has invalid config file path "/Users/MyUser/dev/myproject/xcodegen/MM/Build/xcconfig/conan_config.xcconfig" for config "Debug"