Regexp compilation errors: be more explicit about the failure

Hello,

I was facing the following error during a promises debug session (3.4.5), here is a self-contained example:

/tmp/foo

    [[foo]

bundle.cf

    body common control
    {
        bundlesequence => {
                "regexp",
        };  
    }
     
    bundle agent regexp
    {
      files:
        "/tmp/foo"
          create    =>  "true",
          edit_line =>  my_edit_line;
    }
     
    bundle edit_line my_edit_line {
      replace_patterns:
        "[[foo"
          replace_with => value("replacestring");
      }
     
    body replace_with value(x)
    { 
      replace_value => "$(x)";
    }        

Error message was:

    Regular expression error "missing terminating ] for character class" in expression "[[foo" at 5


The debugging would have been faster if I had the following informations:
  * **missing terminating ] for character class** is a message from libpcre, not CFEngine itself
  * **at 5** is the offset inside the pattern causing the compilation error.

On the current master, the message is more verbose:

    error: /default/regexp/files/'/tmp/foo'/default/my_edit_line: Regular expression error 'missing terminating ] for character class' in expression '[[foo' at 5

I suggest to clarify **at** with **(offset: xx)** and to give a clue about pcre. This way, the advanced user can quickly move to regexp modification (thanks to pcre documentation?), while regular user is not penalized:

    error: /default/regexp/files/'/tmp/foo'/default/my_edit_line: Regular expression error: pcre_compile() 'missing terminating ] for character class' in expression '[[foo' (offset: 5)

But maybe was I too tired, and the error message clear as crystal?

Also, while I'm at it, is it worthwhile to document the flags used internally by **pcre_compile()** in  CompileRegExp() ? 
I mean PCRE_MULTILINE | PCRE_DOTALL, maybe this can be useful to regexp ninjas ? If so, I will issue another PR