[E-ACSL] better messages for unsupported contract clause

src/plugins/e-acsl/TODO

src/plugins/e-acsl/demo/custom_assert.c

src/plugins/e-acsl/doc/Changelog

src/plugins/e-acsl/doc/manuals/e-acsl-manual.pdf

src/plugins/e-acsl/doc/userman/Makefile

src/plugins/e-acsl/doc/userman/biblio.bib

src/plugins/e-acsl/doc/userman/changes.tex

src/plugins/e-acsl/doc/userman/eacslversion.tex

src/plugins/e-acsl/doc/userman/introduction.tex

src/plugins/e-acsl/doc/userman/main.pdf

src/plugins/e-acsl/doc/userman/main.tex

src/plugins/e-acsl/doc/userman/provides.tex

src/plugins/e-acsl/share/e-acsl/e_acsl.c

...s/e-acsl/tests/e-acsl-runtime/oracle/bts1399.1.res.oracle

...ins/e-acsl/tests/e-acsl-runtime/oracle/bts1399.res.oracle

...gins/e-acsl/tests/e-acsl-runtime/oracle/call.1.res.oracle

...lugins/e-acsl/tests/e-acsl-runtime/oracle/call.res.oracle

.../e-acsl/tests/e-acsl-runtime/oracle/localvar.1.res.oracle

...ns/e-acsl/tests/e-acsl-runtime/oracle/localvar.res.oracle

.../e-acsl/tests/e-acsl-runtime/oracle/ptr_init.1.res.oracle