more informative error when processes error in `mcp_tools()`

[simonpcouch]

library(mcptools)

config <- list(
  mcpServers = list(
    "test" = list(
      command = "uv",
      args = c(
        "run",
        "boop"
      )
    )
  )
)

tmpfile <- withr::local_tempfile(fileext = ".json")
jsonlite::write_json(config, tmpfile, auto_unbox = TRUE)

mcp_tools(tmpfile)
#> Error in "process_write_input(self, private, str, sep)": ! Native call to `processx_connection_write_bytes` failed
#> Caused by error in `chain_call(c_processx_connection_write_bytes, con, str)`:
#> ! Cannot write connection (system error 32, Broken pipe) @processx-connection.c:996 (processx_c_connection_write_bytes)

^{Created on 2025-10-22 with reprex v2.1.1}

I wonder if we can surface the actual error raised by the program:

simoncouch@simoncouch-X7FT mcptools % uv run boop
error: Failed to spawn: `boop`
  Caused by: No such file or directory (os error 2)