Improve Crystal::Loader errors

[straight-shoota]

This patch improves error messages from dlopen. They're now also shown for lib arguments (-lfoo) which was missing. Also adds fault tolerance when dlerror returns a null pointer.

/ref #11857

[straight-shoota]

Using String.build here to avoid two intermediary string allocations. Maybe that's overkill for error handling...

[jeffbdavenport]

Trying to test out this code. And I cannot even run crystal i with this branch @asterite @straight-shoota

jeff@jeffs-machine project (master)$ crystal i
In /usr/local/share/crystal/src/prelude.cr:51:1

 51 | require "hash"
      ^
Error: while requiring "hash"


In /usr/local/share/crystal/src/hash.cr:1:1

 1 | require "crystal/hasher"
     ^
Error: while requiring "crystal/hasher"


In /usr/local/share/crystal/src/crystal/hasher.cr:83:27

 83 | Crystal::System::Random.random_bytes(Slice.new(pointerof(@@seed).as(UInt8*), sizeof(typeof(@@seed))))
                              ^-----------
Error: instantiating 'Crystal::System::Random:Module#random_bytes(Slice(UInt8))'


In /usr/local/share/crystal/src/crystal/system/unix/getrandom.cr:41:5

 41 | init unless @@initialized
      ^---
Error: instantiating 'init()'


In /usr/local/share/crystal/src/crystal/system/unix/getrandom.cr:27:8

 27 | if sys_getrandom(Bytes.new(16)) >= 0
         ^------------
Error: instantiating 'sys_getrandom(Slice(UInt8))'


In /usr/local/share/crystal/src/crystal/system/unix/getrandom.cr:93:5

 93 | loop do
      ^---
Error: instantiating 'loop()'


In /usr/local/share/crystal/src/crystal/system/unix/getrandom.cr:93:5

 93 | loop do
      ^---
Error: instantiating 'loop()'


In /usr/local/share/crystal/src/crystal/system/unix/getrandom.cr:94:80

 94 | read_bytes = Syscall.getrandom(buf.to_unsafe, LibC::SizeT.new(buf.size), Syscall::GRND_NONBLOCK)
                                                                               ^---------------------
Error: undefined constant Syscall::GRND_NONBLOCK

[straight-shoota]

Looks as if your master branch is out of sync. This bug was fixed in #11624.