CHOP-951, IQA-1477: Validate args for numeric command-line switches.

The logic in llcommandlineparser.cpp's setControlValueCB() callback function
for converting command-line switch argument values from string to the actual
type of the map-to settings variable had a couple special cases for boolean
and LLSD array. But for S32, U32 and F32, it simply used default LLSD
string-to-numeric conversion. LLSD's string-to-numeric conversion is a bit
lame: for non-numeric strings, it shrugs and returns 0.
Introduce onevalue() and badvalue() helper functions that, like certain errors
during command-line parsing, throw LLCLPError. Use them to streamline certain
redundancies in setControlValueCB(). Introduce convertTo<T>() helper function
that uses boost::lexical_cast() for slightly more stringent conversions. Add
cases for U32, S32 and F32 targets.
setControlValueCB() is actually called only by LLControlGroupCLP::notify(),
not during actual command-line parsing.
Make LLControlGroupCLP::notify() return bool. Make it catch LLCLPError, set
the error for getErrorMessage() and return false on that exception.
Package LLAppViewer::initConfiguration()'s response to initParseCommandLine()
returning false as a new handleCommandLineError() function; invoke it both
there and when LLControlGroupCLP::notify() returns false.