More usable error messages when exceptions are not caught

[Thomas Schneider <qsx@chaotikum.eu>]

Hi,

it would be nice if the error message notmuch shows when an uncaught
exception occurs was more helpful to the user.  It could at least show
the exception message (`e->what()` iirc), which would have helped in
this case.

From IRC:

<qsx> % notmuch new
<qsx> libc++abi: terminating due to uncaught exception of type Xapian::DatabaseError
<qsx> fun
<qsx> does this sound interesting enough to build notmuch and xapian with debug symbols?
[…]
<qsx> but i think the issue is that there’s no space left on the filesystem where the xapian db lives
[…]
<qsx> sounds reasonable
<qsx> but i think it would be nicer for the user if notmuch at least showed the message of the exception, because that was some work to get to this point

Backtrace:

(gdb) p *(Xapian::DatabaseError *)thrown_object
$13 = {<Xapian::RuntimeError> = {<Xapian::Error> = {msg = {static __endian_factor = 2, 
        __r_ = {<std::__1::__compressed_pair_elem<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep, 0, false>> = {__value_ = {{__l = {{__is_long_ = 1, __cap_ = 16}, 
                  __size_ = 25, __data_ = 0x555582cd4fc0 "Error writing block 83232"}, __s = {{__is_long_ = 1 '\001', __size_ = 16 '\020'}, __padding_ = 0x555555c15e61 "", 
                  __data_ = "\000\000\000\000\000\000\000\031\000\000\000\000\000\000\000\300O͂UU\000"}, __r = {__words = {33, 25, 
                    93825755074496}}}}}, <std::__1::__compressed_pair_elem<std::__1::allocator<char>, 1, true>> = {<std::__1::allocator<char>> = {<std::__1::__non_trivial_if<true, std::__1::allocator<char> >> = {<No data fields>}, <No data fields>}, <No data fields>}, <No data fields>}, static npos = 18446744073709551615}, context = {static __endian_factor = 2, 
        __r_ = {<std::__1::__compressed_pair_elem<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep, 0, false>> = {__value_ = {{__l = {{__is_long_ = 0, __cap_ = 0}, 
                  __size_ = 0, __data_ = 0x0}, __s = {{__is_long_ = 0 '\000', __size_ = 0 '\000'}, __padding_ = 0x555555c15e79 "", __data_ = '\000' <repeats 22 times>}, __r = {__words = {0, 0, 
                    0}}}}}, <std::__1::__compressed_pair_elem<std::__1::allocator<char>, 1, true>> = {<std::__1::allocator<char>> = {<std::__1::__non_trivial_if<true, std::__1::allocator<char> >> = {<No data fields>}, <No data fields>}, <No data fields>}, <No data fields>}, static npos = 18446744073709551615}, error_string = {static __endian_factor = 2, 
        __r_ = {<std::__1::__compressed_pair_elem<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep, 0, false>> = {__value_ = {{__l = {{__is_long_ = 0, __cap_ = 0}, 
                  __size_ = 0, __data_ = 0x0}, __s = {{__is_long_ = 0 '\000', __size_ = 0 '\000'}, __padding_ = 0x555555c15e91 "", __data_ = '\000' <repeats 22 times>}, __r = {__words = {0, 0, 
                    0}}}}}, <std::__1::__compressed_pair_elem<std::__1::allocator<char>, 1, true>> = {<std::__1::allocator<char>> = {<std::__1::__non_trivial_if<true, std::__1::allocator<char> >> = {<No data fields>}, <No data fields>}, <No data fields>}, <No data fields>}, static npos = 18446744073709551615}, type = 0x7ffff797410c <str.17.llvm> "\004DatabaseError", my_errno = 28, 
      already_handled = false}, <No data fields>}, <No data fields>}

	--qsx\r