language: raise ShebangDetectionError rather than a generic error

Library/Homebrew/exceptions.rb

@@ -729,3 +729,10 @@ class MacOSVersionError < RuntimeError
     super "unknown or unsupported macOS version: #{version.inspect}"
   end
 end
+
+# Raised when `detected_perl_shebang` etc cannot detect the shebang.
+class ShebangDetectionError < RuntimeError
+  def initialize(type, reason)
+    super "Cannot detect #{type} shebang: #{reason}."
+  end
+end

Library/Homebrew/language/perl.rb

@@ -16,7 +16,7 @@ module Language
         elsif formula.deps.map(&:name).include? "perl"
           Formula["perl"].opt_bin/"perl"
         else
-          raise "Cannot detect Perl shebang: formula does not depend on Perl."
+          raise ShebangDetectionError.new("Perl", "formula does not depend on Perl")
         end
 
         Utils::Shebang::RewriteInfo.new(

Library/Homebrew/language/python.rb

@@ -105,8 +105,10 @@ module Language
       def detected_python_shebang(formula = self)
         python_deps = formula.deps.map(&:name).grep(/^python(@.*)?$/)
 
-        raise "Cannot detect Python shebang: formula does not depend on Python." if python_deps.empty?
-        raise "Cannot detect Python shebang: formula has multiple Python dependencies." if python_deps.length > 1
+        raise ShebangDetectionError.new("Python", "formula does not depend on Python") if python_deps.empty?
+        if python_deps.length > 1
+          raise ShebangDetectionError.new("Python", "formula has multiple Python dependencies")
+        end
 
         python_shebang_rewrite_info(Formula[python_deps.first].opt_bin/"python3")
       end