Don't report warnings when there are errors

All sane compilers do this

compiler/testData/cli/jvm/signatureClash.out

@@ -1,4 +1,3 @@
-WARNING: compiler/testData/cli/jvm/signatureClash.kt: (26, 22) Parameter 'c' is never used
 ERROR: compiler/testData/cli/jvm/signatureClash.kt: (6, 5) Accidental override: The following declarations have the same JVM signature (getX()I):
     fun getX(): kotlin.Int
     fun <get-x>(): kotlin.Int

compiler/testData/cli/jvm/syntheticAccessorSignatureClash.out

@@ -1,4 +1,3 @@
-WARNING: compiler/testData/cli/jvm/syntheticAccessorSignatureClash.kt: (34, 17) Variable 's' is never used
 ERROR: compiler/testData/cli/jvm/syntheticAccessorSignatureClash.kt: (15, 5) Accidental override: The following declarations have the same JVM signature (access$foo$0(LDerived;)V):
     fun `access$foo$0`(d: Derived): kotlin.Unit
     fun foo(): kotlin.Unit

compiler/testData/cli/jvm/wrongAbiVersion.out

@@ -1,5 +1,4 @@
 ERROR: compiler/testData/cli/jvm/wrongAbiVersionLib/bin/ClassWithWrongAbiVersion.class: (0, 0) Class 'ClassWithWrongAbiVersion' was compiled with an incompatible version of Kotlin. Its ABI version is -1, expected ABI version is $ABI_VERSION$
 ERROR: compiler/testData/cli/jvm/wrongAbiVersionLib/bin/wrong/WrongPackage.class: (0, 0) Class 'wrong/WrongPackage' was compiled with an incompatible version of Kotlin. Its ABI version is -1, expected ABI version is $ABI_VERSION$
-WARNING: compiler/testData/cli/jvm/wrongAbiVersion.kt: (3, 9) Parameter 'x' is never used
 ERROR: compiler/testData/cli/jvm/wrongAbiVersion.kt: (4, 3) Unresolved reference: bar
 COMPILATION_ERROR