Mir error messages from exceptions are not user-friendly

Mir error messages from exceptions are not user-friendly.

Test case:
mir_demo_server_shell --vt 123

Expected: Some human-readable explanation about a failure to open VT 123

Observed:
ERROR: /build/buildd/mir-0.8.0+14.10.20141010/src/platform/graphics/mesa/linux_virtual_terminal.cpp(272): Throw in function int mir::graphics::mesa::LinuxVirtualTerminal::open_vt(int)
Dynamic exception type: N5boost16exception_detail10clone_implINS0_19error_info_injectorISt13runtime_errorEEEE
std::exception::what: Failed to open current VT
2, "No such file or directory"[PN5boost18errinfo_file_name_E] = /dev/tty123

What shouldn't be visible: File paths, mangled symbol names, double-colons or the words "throw" or "exception". These are all words useful to programmers, but our target audience for the general activity of starting a shell is users, not programmers.

What should be visible: Information that's meaningful without any knowledge of the Mir source code; Some mention of "Failed to open VT", and maybe "/dev/tty123", maybe "No such file or directory".