fix: report formatting errors with line numbers

[jamietanna]

When formatting the resulting output, using goimports, we may
receive an error due to generating invalid code using our templates.

The output returned is fairly unhelpful, noting a line number, but
requiring the users to take the broken output and then paste it into
something that has a line number.

This improves the output by reporting:

• line numbers for the resulting Go code
• a ❗ where the (first) formatting error is found

---

I.e.

    104:             }
    105:             a.AdditionalProperties[fieldName] = fieldVal
    106:         }
    107:     }
    108:        return nil
    109: }
    110: 
    111: // Override default JSON handling for ArrayContainer to handle AdditionalProperties
    112: func (a ArrayContainer) MarshalJSON() ([]byte, error) {
    113:     var err error
    114:     object := make(map[string]json.RawMessage)
    115: 
    116:     /* JVT:
    117:     #v%!(EXTRA codegen.Property={ values {[]string  0xc0000de600 map[] [] false <nil> [] true  [] <nil> true 0xc0000e0c00} false false false false false map[x-go-type-skip-optional-pointer:true] false})
    118:     #{ values {[]string  0xc0000de600 map[] [] false <nil> [] true  [] <nil> true 0xc0000e0c00} false false false false false map[x-go-type-skip-optional-pointer:true] false}
    119:     []string
    120:     []string
    121: 
    122:     */
    123: if a.Values != nil { 
    124:     object["values"], err = json.Marshal(a.Values)
    125:     if err != nil {
    126:         return nil, fmt.Errorf("error marshaling 'values': %w", err)
    127:     }
    128: 
    129: 
    130:     for fieldName, field := range a.AdditionalProperties {
    131:                object[fieldName], err = json.Marshal(field)
    132:                if err != nil {
    133:                        return nil, fmt.Errorf("error marshaling '%s': %w", fieldName, err)
    134:                }
    135:        }
    136:        return json.Marshal(object)
    137: }
    138: 
❗  139: 
    140: 
error was: issue2031.go:139:2: expected '}', found 'EOF'
exit status 1
generate.go:3: running "go": exit status 1