Usability bug: Add accurate error message for mcl scripts

[dbakong]

Versions:

• mgmt version (eg: mgmt --version): 0.0.20

• operating system/distribution (eg: uname -a): fedora 30

• golang version (eg: go version): go1.12.7

Description:

The error message when running an mcl script with a non-existent field needs to be fixed.

mcl file:

$noop = false
exec "exec1" {
    cmd => "pwd",
    shell => "/bin/bash",
    Meta:autoedge => true,
    group => "group1",
    gid => 110,
}

user "user1" {
    state => "exists",
    Meta:autoedge => true,
    group => "group1",
}

user "user2" {
    state => "exists",
    group => "group2",
    Meta:autoedge => true,
}

group "group1" {
    state => "exists",
    gid => 110,
    Meta:autoedge => true,
}

group "group2" {
    state => "exists",
    gid => 111,
    Meta:autoedge => true,
}

log of failed run:

2019-09-07 22:13:46.852235 I | cli: lang: lexing/parsing...
2019-09-07 22:13:46.872282 I | cli: lang: init...
2019-09-07 22:13:46.872368 I | cli: lang: interpolating...
2019-09-07 22:13:46.872489 I | cli: lang: building scope...
2019-09-07 22:13:46.872911 I | cli: lang: running type unification...
2019-09-07 22:13:46.873209 I | run: error: cli parse error: could not unify types: could not determine type for `gid` field of `exec`

[purpleidea]

I actually already patched this in a feature branch. Coming to git master shortly. Thanks!

[purpleidea]

Fixed in gitmaster.