title: CMake multiline message with FATAL_ERROR
body: <p>CMake documentation (for example current <a href="https://cmake.org/cmake/help/v3.11/command/message.html" rel="noreferrer">version 3.11.2</a>) states</p>
  
  <blockquote>
    <p>CMake Warning and Error message text displays using a simple markup language. Non-indented text is formatted in line-wrapped paragraphs delimited by newlines. Indented text is considered pre-formatted.</p>
  </blockquote>
  
  <p>However, it doesn't mention any markup format. Unless the "non-indented" vs. "indented" is all there is about the "simple markup".</p>
  
  <p>Anyway, I failed to make it work with <code>FATAL_ERROR</code> mode.</p>
  
  <p>Furthermore, I noticed that with <code>STATUS</code> mode message is printed with leading <code>--</code> (two dashes and space). While with <code>FATAL_ERROR</code> every line break in the message is turned into two lines, which (IMHO) looks awful.</p>
  
  <p>Now I have a multiline message which lists what is wrong in <code>CMAKE_BUILD_TYPE</code> and what values are accepted. Because of above-mentioned issues, I ended up printing the message as <code>STATUS</code> and indenting subsequent lines with three spaces (so they align well with the <code>--</code>). Then I do a simple <code>FATAL_ERROR</code> repeating only the "title line" (stating that <code>CMAKE_BUILD_TYPE</code> is wrong). This looks acceptable on both console output and <code>cmake-gui</code>. (Although the 3 spaces indentation is needless on <code>cmake-gui</code>...)</p>
  
  <p>However, I'm surprised how poorly is this topic described. And it seems to be so since long - see for example question <a href="https://cmake.org/pipermail/cmake/2009-October/032448.html" rel="noreferrer">[CMake] Extra blank lines with SEND_ERROR and FATAL_ERROR ?!</a> remaining unanswered for almost 9 years now...</p>
  
  <p>Are there any good practices, advice or tips for handling such messages? Or should they be avoided in the first place?</p>