Improve error message when parttest fails due to missing pdflatex

Summary:
`PartTest::testForwardPDF` tests whether the synctex integration works
and needs to create a PDF from latex sources via `pdflatex`.
If this is not installed the test fails, but the error message
`'started' returned FALSE` requires diving into the source code to debug
the problem.

By providing a more meaningful error message we make it easier for
developers and distributions to run all autotests sucessfully (short of
build-requiring a latex installation). Note that Albert does not want to
simply skip the tests in that case, so no optional build-time dependency
either.

Ref T7532

Test Plan:
Run `parttest testForwardPDF` w/ and w/o `pdflatex` executable
available.

Reviewers: aacid, #okular

Reviewed By: aacid, #okular

Subscribers: #okular

Tags: #okular

Maniphest Tasks: T7532

Differential Revision: https://phabricator.kde.org/D9578