nimble shows errors from conditional compilation `when compiles(...)` 
test case: see https://github.com/nim-lang/nimble/issues/608

the `Error` below point to `when compiles(...)` statements which should gag (inhibit) errors instead of showing them; eg:

```
/Users/timothee/git_clone/nim/Nim/lib/core/macros.nim(689, 15):
  when compiles(float128):
/Users/timothee/git_clone/nim/Nim/lib/system.nim(2724, 67)
    when value isnot string and value isnot seq and compiles(value.isNil):
```

log showing non-gagged errors:
```
nimble install --verbose -y
    Reading config file at /Users/timothee/.config/nimble/nimble.ini
    Setting Nim stdlib prefix to
    Setting Nim stdlib path to /Users/timothee/git_clone/nim/Nim/lib
       Info Hint: used config file '/Users/timothee/git_clone/nim/Nim/config/nim.cfg' [Conf]
       Info Hint: used config file '/Users/timothee/.config/nim/nim.cfg' [Conf]
       Info /Users/timothee/git_clone/nim/Nim/lib/core/macros.nim(689, 15) Error: undeclared identifier: 'float128'
       Info /Users/timothee/git_clone/nim/Nim/lib/core/macros.nim(1030, 44) template/generic instantiation of `$` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/system.nim(2741, 21) template/generic instantiation of `collectionToString` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/system.nim(2724, 67) Error: type mismatch
       Info /Users/timothee/git_clone/nim/Nim/lib/core/macros.nim(1030, 44) template/generic instantiation of `$` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/system.nim(2741, 21) template/generic instantiation of `collectionToString` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/system.nim(2731, 13) template/generic instantiation of `addQuoted` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/system.nim(4205, 18) Error: type mismatch
       Info /Users/timothee/git_clone/nim/nimterop/nimterop.nimble(28, 11) template/generic instantiation of `&` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/pure/strformat.nim(261, 23) Error: type mismatch
       Info /Users/timothee/git_clone/nim/nimterop/nimterop.nimble(28, 11) template/generic instantiation of `&` from here
       Info /Users/timothee/git_clone/nim/Nim/lib/pure/strformat.nim(263, 39) Error: type mismatch
       Info /Users/timothee/git_clone/nim/nimterop/nimterop.nimble(28, 11) template/generic instantiation of `&` from here
  Verifying dependencies for nimterop@0.1.0
    Reading official package list
   Checking for regex@>= 0.10.0
      Info: Dependency on regex@>= 0.10.0 already satisfied
    ....
   # evenutally succeeds
```

* note: this can occur when using `strformat` inside a nimble file
* note: `nims` doesn't seem to have this problem, eg:


```
import strformat
let bar = 23
echo "foo{bar}"
```

```
nim e --verbosity:2 $timn_D/tests/nim/all/t0145.nims
# no such error shown
```