Repro:
- A person moves a test data directory and doesn't update BUILD.gn
- Due to incremental build, this doesn't fail right away, passes the CQ; or this is for an configuration outside of the CQ (e.g. perf)
- Breakage on clobber build after the fact after compile in the isolate step, which users aren't fully familiar with.

Issue:
- The fact that 'isolate tests' fails makes it look like an infrastructure issue.
- The error message is unclear about what is happening during isolate archive, e.g.
"14:39:45.846558 Unable to walk "/b/build/slave/Android_Compile/build/src/cc/test/data": walk("/b/build/slave/Android_Compile/build/src/cc/test/data"): lstat /b/build/slave/Android_Compile/build/src/cc/test/data: no such file or directory"
as seen in  issue 746016 

Observations about the error message:
- lstat() part isn't relevant to the user
- path is listed twice, which makes the error message hard to read
- 'Unable to walk' is kind of obscure
- path is absolute instead of relative, which hinders the understanding

AIs:
- Improve the error message to give a nudge about the fact that an input directory was listed but it's not present. The fact that it was found via lstat isn't relevant to the user and the path should be relative as the user may not know what the base directory is.
- This should be printed as an error, not as a log which drowns it into the noise.