improve TOML syntax error reporting

Description

TOML syntax error reports are not informative enough even when starting the runner in --debug mode. Example:

gitlab-ci-multi-runner --debug -l debug verify
Runtime platform                                    arch=amd64 os=linux revision=a3543a27 version=10.6.0
Checking runtime mode                               GOOS=linux uid=0
Running in system-mode.

FATAL: toml: cannot load TOML value of type map[string]interface {} into a Go slice

Based on this error message it is only possible to find a particular error using a trial-and-error approach.

Proposal

When reporting TOML syntax errors, please include the line number and the option causing the error.