Provide better feedback on invalid compiler field WAS Rejects valid compiler field

Does not support the following valid compiler value:

compiler: 8.8.0.20190613

### Steps to reproduce

Run:

stack build

with this `stack.yaml`:

```
allow-newer: true

#compiler: ghc-8.6.5
#compiler: ghc-8.8.1
#compiler: 8.8.1-alpha2
compiler: 8.8.0-20190613

compiler-check: match-exact
resolver: lts

setup-info:
ghc:
linux32:
8.8.0.20190613:
url: "https://downloads.haskell.org/~ghc/8.8.1-alpha2/ghc-8.8.0.20190613-i386-deb9-linux.tar.xz"
content-length: 204060792
sha1: 0fe7fc95073602078e0677ae623dd02f46ae3a5b
sha256: 77d23afaf035775c99b48f0c78117a8073682dde3c6104b8ced4d5d062287772
macosx:
8.8.0.20190613:
url: "https://downloads.haskell.org/~ghc/8.8.1-alpha2/ghc-8.8.0.20190613-x86_64-apple-darwin.tar.xz"
content-length: 184846152
sha1: 116bdaadcde045f78adcf1ffce291c79fb1c3eb6
sha256: 04b5b4dc35db2bc26f5e4d46946409a5d21f47a7923bf69ee31b093b7e71e889
linux64:
8.8.0.20190613:
url: "https://downloads.haskell.org/~ghc/8.8.1-alpha2/ghc-8.8.0.20190613-x86_64-deb9-linux.tar.xz"
content-length: 199898532
sha1: 11360a17f61da02c82d0f64f07e124479cedecc4
sha256: ff532d0b700fcd9c5343a942eab35450cd9d70ce5c5064ca4dfe7af94b05616e
linux64-tinfo6:
8.8.0.20190613:
url: "https://downloads.haskell.org/~ghc/8.8.1-alpha2/ghc-8.8.0.20190613-x86_64-fedora27-linux.tar.xz"
content-length: 201560804
sha1: 07eb8ab473f2aa3c21ac2b497142a01466ace5ee
sha256: cf972265333244c8648b99fd1bcf90e1a3e7b5335c0a418f6bc9849382de8f48
windows64:
8.8.0.20190613:
url: "https://downloads.haskell.org/~ghc/8.8.1-alpha2/ghc-8.8.0.20190613-x86_64-unknown-mingw32.tar.xz"
content-length: 224468920
sha1: b4a3b2b45625eb32dd7f892246da9a602d9ce7db
sha256: 83b4c89d01c711c0f64bcc5abda287678c027b1b0d44ada17036917aeae588eb
```

### Expected

Try to build with the alpha version of GHC 8.8

### Actual

Fails to parse with an error message that does not provide the user with the requisite information to correct the issue at hand.

### Stack version

```
Version 2.1.1, Git revision f612ea85316bbc327a64e4ad8d9f0b150dc12d4b (7648 commits) x86_64 hpack-0.31
```

### Method of installation

Don't remember. It was like 3 years and a half dozen upgrades ago.