[Tex/LaTex] How to cut a section title in the header

fancyhdrheader-footerxstring

I redefine my headers as in the following MWE:

\documentclass[12pt]{article}
\usepackage{xstring}
\usepackage{fancyhdr}
\fancypagestyle{monstyle}{%
\fancyhf{} % remove everything
\renewcommand{\headrule}{\rule[1.5ex]{\headwidth}{1pt}}
  \lhead{%
\leftmark%
  }
}
\begin{document}
\pagestyle{monstyle}
\section{le tres tres long titre qui prend trop de place sur la ligne}
\end{document}

The section title could be very long, like in my MWE, and i want to cut it at a fixed length (say 15 letters). My attempt is to use the command \StrLeft from the package xstring : I replace the line 

\leftmark%

by 

    \StrLeft{\leftmark}{15}...%

But i get compilation errors which i do not understand:

Argument of \@iiparbox has an extra }.

EDIT
I try to use Mico's solution, but i encounter now a problem with the TOC : 

\documentclass[12pt]{article}
\usepackage{xstring}
\usepackage{fancyhdr}
\usepackage{blindtext}

%% Redefine the short title
\let\origsection\section % 
\renewcommand{\section}[1]{% 
\def\ShortSecName{\StrLeft{#1}{10}...}
\origsection[\protect\ShortSecName]{#1}%
}

\begin{document}
\tableofcontents
\section{mon titre tres long tres tres long}
\blindtext\blindtext\blindtext
\section{mon titre tres long tres tres long}
\blindtext\blindtext\blindtext
\end{document}

I get the error : 

! LaTeX Error: Something's wrong--perhaps a missing \item.

If i remove \tableofcontents then it compiles.

Best Answer

An alternative approach would be using titleps instead of fancyhdr (caveat: I'm the author of titleps):

\documentclass[12pt]{article}
\usepackage{xstring}
\usepackage{titleps}

\newpagestyle{monstyle}{
   \headrule
   \sethead{\thepage}
           {}
           {\StrLeft{\sectiontitle}{15}...}}

\begin{document}
\pagestyle{monstyle}
\section{le tres tres long titre qui prend trop de place sur la ligne}
\end{document}

\sectiontitle is a macro containing just the title, not a mark with part of the header (formatted), and therefore it can be handled easily.

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