Handle cases where user ID does not map to a user name (e.g. docker)

[arnaudh]

Bumped into this issue while working within a docker container:

julia> stat(Path("/test/foo.txt"))
ERROR: ArgumentError: User 1000 not found.
Stacktrace:
 [1] FilePathsBase.User(uid::UInt64)
   @ FilePathsBase ~/.julia/packages/FilePathsBase/YFK4h/src/libc.jl:84
 [2] Status(s::Base.Filesystem.StatStruct)
   @ FilePathsBase ~/.julia/packages/FilePathsBase/YFK4h/src/status.jl:21
 [3] stat(fp::PosixPath)
   @ FilePathsBase ~/.julia/packages/FilePathsBase/YFK4h/src/system.jl:54
 [4] top-level scope
   @ REPL[5]:1

This happens because the owner of the file in question has uid 1000, and that particular user is not visible inside docker. This answer on Stack Overflow lists other possible use cases: mounting an external file system, or a file belonging to a deleted user.
It would be nice if FilePathsBase supported those use cases, i.e. if it were possible to manipulate files even if the owners or groups can't be found.

Here's a Minimum Working Example to reproduce:

# On an AWS EC2 instance running linux:

-bash-4.2$ mkdir test && cd test

-bash-4.2$ touch foo.txt

-bash-4.2$ ls -l
total 0
-rw-rw-r-- 1 ec2-user ec2-user 0 Sep  7 16:27 foo.txt

-bash-4.2$ ls -ln # Display user and group IDs numerically
total 0
-rw-rw-r-- 1 1000 1000 0 Sep  7 16:27 foo.txt

-bash-4.2$ docker run -it -v $(pwd):/test julia:1.6 bash

root@fdad1f65434d:$ ls -l /test # Inside docker we see the user id doesn't get translated to a username
total 0
-rw-rw-r-- 1 1000 1000 0 Sep  7 16:27 foo.txt

root@fdad1f65434d:$ julia
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.6.2 (2021-07-14)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

(@v1.6) pkg> add FilePathsBase
  Installing known registries into `~/.julia`
       Added registry `General` to `~/.julia/registries/General`
   Resolving package versions...
   Installed FilePathsBase ─ v0.9.10
    Updating `~/.julia/environments/v1.6/Project.toml`
  [48062228] + FilePathsBase v0.9.10
    Updating `~/.julia/environments/v1.6/Manifest.toml`
  [48062228] + FilePathsBase v0.9.10
  [2a0f44e3] + Base64
  [ade2ca70] + Dates
  [b77e0a4c] + InteractiveUtils
  [56ddb016] + Logging
  [d6f4376e] + Markdown
  [a63ad114] + Mmap
  [de0858da] + Printf
  [9a3f8284] + Random
  [ea8e919c] + SHA
  [9e88b42a] + Serialization
  [8dfed614] + Test
  [cf7118a7] + UUIDs
  [4ec0a83e] + Unicode
Precompiling project...
  1 dependency successfully precompiled in 3 seconds

julia> run(`ls -l /test`)
total 0
-rw-rw-r-- 1 1000 1000 0 Sep  7 16:27 foo.txt
Process(`ls -l /test`, ProcessExited(0))

julia> using FilePathsBase

julia> stat(Path("/test/foo.txt"))
ERROR: ArgumentError: User 1000 not found.
Stacktrace:
 [1] FilePathsBase.User(uid::UInt64)
   @ FilePathsBase ~/.julia/packages/FilePathsBase/YFK4h/src/libc.jl:84
 [2] Status(s::Base.Filesystem.StatStruct)
   @ FilePathsBase ~/.julia/packages/FilePathsBase/YFK4h/src/status.jl:21
 [3] stat(fp::PosixPath)
   @ FilePathsBase ~/.julia/packages/FilePathsBase/YFK4h/src/system.jl:54
 [4] top-level scope
   @ REPL[5]:1

[rofinn]

Hmm, I think other languages (python) also error in this case, but I don't see any obvious security issues with throwing a warning and pushing the issue down the line. I think #154 should resolve the immediate issue, though depending on your permissions might still not be able to modify the file.