Fix compile error message not printed to correct writer on error

[silvanocerza]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requestsx
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fixes an issue with compile command and related gRPC function.

• What is the current behavior?

If the user tries to compile a Sketch that includes a library that's not installed an error is printed to stderr.

The gRPC interface doesn't show the error at all.

$ arduino-cli compile -b arduino:avr:uno /home/alien/Arduino/Blink/Blink.ino --format json
/home/alien/Arduino/Blink/Blink.ino:1:10: fatal error: SD: No such file or directory
 #include <SD>
          ^~~~
compilation terminated.
{
  "compiler_out": "",
  "compiler_err": "",
  "builder_result": {
    "build_path": "/tmp/arduino-sketch-302F70E5AC1B118F7324375770CFA163"
  },
  "success": false
}

• What is the new behavior?

Compilation of a Sketch that includes a library not installed now correctly prints the error to the expected error stream.
This also fixes the gRPC function.

$ arduino-cli compile -b arduino:avr:uno /home/alien/Arduino/Blink/Blink.ino --format json
{
  "compiler_out": "",
  "compiler_err": "/home/alien/Arduino/Blink/Blink.ino:1:10: fatal error: SD: No such file or directory\n #include \u003cSD\u003e\n          ^~~~\ncompilation terminated.\n",
  "builder_result": {
    "build_path": "/tmp/arduino-sketch-302F70E5AC1B118F7324375770CFA163"
  },
  "success": false
}

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

None.

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