Toolchain should be removed on build fail
```
dom@mail:~$ choosenim \#async-timers-improvements --verbose
Downloading Nim async-timers-improvements from GitHub
      Info: /home/dom/.choosenim/downloads/async-timers-improvements.tar.gz already downloaded
 Extracting async-timers-improvements.tar.gz
   Building Nim #async-timers-improvements
   Warning: Building from latest C sources. They may not be compatible with the Nim version you have chosen to install.
      Info: /home/dom/.choosenim/downloads/master.tar.gz already downloaded
 Extracting master.tar.gz
   Building C sources
   Building koch
 Exception: choosenim.nim(206)       choosenim
        ... choosenim.nim(187)       performAction
        ... choosenim.nim(89)        choose
        ... choosenim.nim(72)        chooseVersion
        ... choosenim.nim(46)        installVersion
        ... builder.nim(91)          build
        ... builder.nim(43)          buildCompiler
        ... utils.nim(19)            doCmdRaw
        ... Execution failed with exit code 1
        ... Command: ./bin/nim c koch
        ... Output: Hint: used config file '/home/dom/.choosenim/toolchains/nim-#async-timers-improvements/config/nim.cfg' [Conf]
        ... Hint: used config file 'koch.nim.cfg' [Conf]
        ... Hint: system [Processing]
        ... lib/system.nim(468, 35) Warning: unknown magic 'Exception' might crash the compiler [UnknownMagic]
        ... lib/system/sysstr.nim(110, 25) Error: type mismatch: got (ptr UncheckedArray[system.char]) but expected 'cstring = CString'
     Error: choosenim.nim(206)       choosenim
        ... choosenim.nim(187)       performAction
        ... choosenim.nim(89)        choose
        ... choosenim.nim(72)        chooseVersion
        ... choosenim.nim(46)        installVersion
        ... builder.nim(100)         build
        ... Build failed
dom@mail:~$ rm -r /home/dom/.choosenim/downloads/master.tar.gz
dom@mail:~$ choosenim \#async-timers-improvements --verbose
   Warning: Could not find toolchain's Nimble version.
   Switched to Nim #async-timers-improvements
```