The LLTHROW() abstraction allows us to enrich the subject exception with a
boost::stacktrace -- without having to propagate the boost/stacktrace.hpp
header throughout the code base.

To my delight, our existing use of
boost::current_exception_diagnostic_information() already reports the newly
added boost::stacktrace information -- we don't have to query it specifically!

Wrap coroutine call in try/catch in top-level coroutine wrapper function
LLCoros::toplevel(). Distinguish exception classes derived from
LLContinueError (log and continue) from all others (crash with LL_ERRS).

Enhance CRASH_ON_UNHANDLED_EXCEPTIONS() and LOG_UNHANDLED_EXCEPTIONS() macros
to accept a context string to supplement the log message. This lets us replace
many places that called boost::current_exception_diagnostic_information() with
LOG_UNHANDLED_EXCEPTIONS() instead, since the explicit calls were mostly to
log supplemental information.

Provide supplemental information (coroutine name, function parameters) for
some of the previous LOG_UNHANDLED_EXCEPTIONS() calls. This information
duplicates LL_DEBUGS() information at the top of these functions, but in a
typical log file we wouldn't see the LL_DEBUGS() message.

Eliminate a few catch (std::exception e) clauses: the information we get from
boost::current_exception_diagnostic_information() in a catch (...) clause
makes it unnecessary to distinguish.

In a few cases, add a final 'throw;' to a catch (...) clause: having logged
the local context info, propagate the exception to be caught by higher-level
try/catch.

In a couple places, couldn't resist reconciling indentation within a
particular function: tabs where the rest of the function uses tabs, spaces
where the rest of the function uses spaces.

In LLLogin::Impl::loginCoro(), eliminate some confusing comments about an
array of rewritten URIs that date back to a long-deleted implementation.

since Visual Studio doesn't know __PRETTY_FUNCTION__, and Boost already has a
portable macro to Do The Right Thing.

Turns out we have a surprising number of catch (...) clauses in the viewer
code base. If all we currently do is

    LL_ERRS() << "unknown exception" << LL_ENDL;

then call CRASH_ON_UNHANDLED_EXCEPTION() instead. If what we do is

    LL_WARNS() << "unknown exception" << LL_ENDL;

then call LOG_UNHANDLED_EXCEPTION() instead.

Since many places need LOG_UNHANDLED_EXCEPTION() and nobody catches
LLContinueError yet, eliminate LLContinueError& parameter from
LOG_UNHANDLED_EXCEPTION(). This permits us to use the same log message as
CRASH_ON_UNHANDLED_EXCEPTION(), just with a different severity level.

Where a catch (...) clause actually provides contextual information, or makes
an error string, add boost::current_exception_diagnostic_information() to try
to figure out actual exception class and message.

A level of preprocessor indirection lets us later change the implementation if
desired.

llexception_test.cpp is an unusual test source in that it need not be verified
on every build, so its invocation in indra/llcommon/CMakeLists.txt is
commented out with that remark. Its purpose is to help a developer decide what
base class(es) to use for LLException, how to throw and how to catch.

Our current conclusions are written up as comments in llexception_test.cpp.

Added CRASH_ON_UNHANDLED_EXCEPTION() and LOG_UNHANDLED_EXCEPTION() macros to
llexception.h -- macros to log __FILE__, __LINE__ and __PRETTY_FUNCTION__ of
the catch site. These invoke functions in llexception.cpp so we don't need to
#include llerror.h for every possible catch site.

This also introduces LLContinueError for exceptions which should interrupt
some part of viewer processing (e.g. the current coroutine) but should attempt
to let the viewer session proceed.

Derive all existing viewer exception classes from LLException rather than from
std::runtime_error or std::logic_error.

Use BOOST_THROW_EXCEPTION() rather than plain 'throw' to enrich the thrown
exception with source file, line number and containing function.