Fix error message for invalid shard.yml (#516)

Author: straight-shoota

spec/integration/install_spec.cr

@@ -954,6 +954,14 @@ describe "install" do
     end
   end
 
+  it "shows error when installing by ref and spec is invalid" do
+    metadata = {dependencies: {invalidspec: {git: git_url(:invalidspec), tag: "v0.1.0"}}}
+    with_shard(metadata) do
+      ex = expect_raises(FailedCommand) { run "shards install --no-color" }
+      ex.stdout.should contain(%(E: Invalid shard.yml for shard "invalidspec" at commit #{git_commits(:invalidspec)[0]}: Expected SCALAR but was SEQUENCE_START at line 5, column 1))
+    end
+  end
+
   it "install latest version despite current crystal being older version, but warn" do
     metadata = {dependencies: {incompatible: "*"}}
     with_shard(metadata) do

spec/integration/spec_helper.cr

@@ -62,6 +62,9 @@ private def setup_repositories
   create_git_release "noshardyml", "0.1.0", false
   create_git_release "noshardyml", "0.2.0"
 
+  create_git_repository "invalidspec"
+  create_git_release "invalidspec", "0.1.0", {crystal: [""]}
+
   # dependencies with postinstall scripts:
   create_git_repository "post"
   {% if flag?(:win32) %}

src/resolvers/git.cr

@@ -152,15 +152,18 @@ module Shards
       end
     end
 
-    private def spec_at_ref(ref : GitRef) : Spec?
+    private def spec_at_ref(ref : GitRef, commit) : Spec
       update_local_cache
+
+      unless file_exists?(ref, SPEC_FILENAME)
+        raise Error.new "No #{SPEC_FILENAME} was found for shard #{name.inspect} at commit #{commit}"
+      end
+
+      spec_yaml = capture("git show #{Process.quote("#{ref.to_git_ref}:#{SPEC_FILENAME}")}")
       begin
-        if file_exists?(ref, SPEC_FILENAME)
-          spec_yaml = capture("git show #{Process.quote("#{ref.to_git_ref}:#{SPEC_FILENAME}")}")
-          Spec.from_yaml(spec_yaml)
-        end
-      rescue Error
-        nil
+        Spec.from_yaml(spec_yaml)
+      rescue error : Error
+        raise Error.new "Invalid #{SPEC_FILENAME} for shard #{name.inspect} at commit #{commit}: #{error.message}"
       end
     end
 
@@ -183,11 +186,8 @@ module Shards
         raise Error.new "Could not find #{ref.full_info} for shard #{name.inspect} in the repository #{source}"
       end
 
-      if spec = spec_at_ref(ref)
-        Version.new "#{spec.version.value}+git.commit.#{commit}"
-      else
-        raise Error.new "No #{SPEC_FILENAME} was found for shard #{name.inspect} at commit #{commit}"
-      end
+      spec = spec_at_ref(ref, commit)
+      Version.new "#{spec.version.value}+git.commit.#{commit}"
     end
 
     def matches_ref?(ref : GitRef, version : Version)