`--line` should fail when no proof obligation is selected

[lemmy]

When tlapm --line <n> is given a line that does not select any proof obligation, TLAPM exits successfully and reports that all
zero obligations were proved. This can make an off-by-one line number look like a successful proof.

Reproducer

---- MODULE true_test ----
EXTENDS TLAPS
THEOREM TRUE
    OBVIOUS
====

$ tlapm --line 1 true_test.tla
@!!BEGIN
@!!type:obligationsnumber
@!!count:0
@!!END
File "./true_test.tla", line 1, character 1 to line 8, character 4:
[INFO]: All 0 obligation proved.
$ echo $?
0

Proposed behavior

For an explicit target such as --line, selecting zero obligations should return a nonzero exit code,
e.g.

[WARNING]: No proof obligation found at line 1.

A full-module run with genuinely zero obligations can still succeed. The proposed change is only for explicit proof targets
(--line, and possibly targeted Toolbox ranges).

Environment

$ tlapm --version
fd3988f

Potentially related: #245